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  • IMPERIAL COLLEGE LONDON Department of Earth Science and Engineering Centre for Petroleum Studies Shallow Seismic Analysis in Pagosa Springs, Colorado, USAby Junghee KimA report submitted in partial fulfilment of therequirements for the MScSeptember 2012DECLARATION OF OWN WORKI declare that this thesis is entirely my own work and that where any material could beconstrued as the work of others, this has been fully cited and referenced, and/or withappropriate acknowledgement given. Signature Name of student Junghee Kim Name of supervisor Dr. Adam Booth Word Count 14744 words
  • ABSTRACTIn the Pagosa Springs, Colorado USA, students of Imperial College London and ColoradoSchool of Mines Geophysics Camp 2012 have performed geophysical analyses. Seismicdata, comprising P-wave and S-wave data acquired along two lines (North Line and ZenGarden), were interpreted to analyse near surface geology for geotechnical and groundwaterpurposes.Refraction analyses were performed using gradient-intercept, reciprocal, time term inversionand tomographic inversion methods to calculate the velocity and thickness of eachsubsurface layer. The presence of significant refractor overlaps favoured reciprocal and timeterm inversion methods as it allowed enough room for delay time window analysis to beperformed.Results of each of these methods show a strong correlation in velocity and thickness values.Output of the time term inversion was fed into the tomographic inversion as a starting model.Convergence to a local minimum was reached after about 10 iterations, with an RMS error ofless than 10% in most cases.Analyses of the results in the North Line and Zen Garden area show a slightly undulatingthree layer near surface geology with a dip. Unconsolidated sediments with depth of about 2m and properties that are consistent with shale were interpreted. The layer occupying adepth between 2 m to around 15 m was interpreted to be water saturated sandstone. Thedepth over 15 m seems like sandstone. However because the depth over 15 m is notreachable with ray tracing path, it is not possible to sample beyond ~15 m with the hammerseismic data.By using the velocities acquired from tomographic inversion, datum static correction(including refraction static correction) has been performed to the reflection data, after stackand show improvement in terms of continuity of reflectivity. However, it suffers frominsensitivity due to its very shallow features.Junghee Kim 1
  • Table of ContentsABSTRACT............................................................................................................................................ 1ACKNOWLEDGEMENT ...................................................................................................................... 9CHAPTER ONE ................................................................................................................................... 101.0 Introduction ............................................................................................................................... 101.1 Objectives ....................................................................................................................................... 11CHAPTER TWO .................................................................................................................................. 122.0 Geological setting of Pagosa Springs, Colorado USA .............................................................. 12CHAPTER THREE .............................................................................................................................. 153.0 Theory and Literature review .................................................................................................... 153.1 Refraction Seismic Method ....................................................................................................... 153.2 Time-Distance curves for layered media .................................................................................. 163.3 Hidden Layers, Velocity Inversions, and Blind Zones ............................................................. 203.4 Refraction Arrival picking and time adjustments ..................................................................... 223.5 Manual picking and automatic picking of traveltimes .............................................................. 223.6 Reciprocal Time Correlation ..................................................................................................... 233.7 Refraction Interpretation ........................................................................................................... 243.8 Gradient-Intercept method ........................................................................................................ 243.9 Delay-Time Concept ................................................................................................................. 243.10 Reciprocal Method ........................................................................................................................ 263.11 Term-time inversion.................................................................................................................. 313.12 Tomographic inversion method .................................................................................................... 35CHAPTER FOUR................................................................................................................................. 394.0 METHODOLOGY ................................................................................................................... 394.1 Data acquisition ........................................................................................................................ 404.3 Refraction Data Analysis .......................................................................................................... 46 4.3.1 Basic refraction analysis in North Line.................................................................................... 46 4.3.1.1 Promax .................................................................................................................................. 46 4.3.1.2 Geometry assignment............................................................................................................ 46 4.3.1.3 Initial data analysis and quality control ................................................................................ 47 4.3.1.4 First Break Picking in Promax .............................................................................................. 47 4.3.1.5 Extraction to Matlab ............................................................................................................. 48 4.3.1.6 Gradient intercept method ..................................................................................................... 49Junghee Kim 2
  • 4.3.2 Advanced refraction analysis (North Line) ............................................................................ 50 4.4.2.1 Seisimager ............................................................................................................................. 50 4.4.2.2 Initial data analysis and quality control ................................................................................ 50 4.4.2.3 Data Processing ..................................................................................................................... 50 4.4.2.4 Elevation importing. ............................................................................................................. 50 4.4.2.5 Amplitude Recovery ............................................................................................................. 51 4.4.2.6 Travel Time Pick and QC ..................................................................................................... 52 4.4.2.7 Reciprocal Time Check......................................................................................................... 52 4.4.2.8 First break picks of P-wave in North Line ............................................................................ 53 4.4.2.9 Advanced Seismic Refraction Analysis using Seisimager .................................................... 53 4.4.2.10 Layer assignment ................................................................................................................ 53 4.4.2.11 Reciprocal method .............................................................................................................. 54 4.4.2.12 Time term inversion ............................................................................................................ 55 4.4.2.13 Tomographic inversion ....................................................................................................... 56 4.3.3 Seismic Reflection Data Processing and Analysis in North Line ............................................ 60 4.3.3.1 Refraction Muting ................................................................................................................. 60 4.3.3.2 Bandpass Filtering ................................................................................................................ 62 4.3.3.3 Static Correction ................................................................................................................... 64 4.3.3.3.1 Elevation Statics Analysis in North line. ........................................................................... 65 4.3.3.3.2 Datum static correction from tomographic inversion of Seisimager in Promax: ............... 66 4.3.3.4 Stacking................................................................................................................................. 68 4.3.4 Comparison with the other methods (DC-resistivity) .............................................................. 71 4.3.4.1 DC Resistivity Survey........................................................................................................... 71 4.3.5 Advanced refraction analysis (Zen Garden ) ........................................................................... 73 4.3.5.1 First break picks of P-wave in Zen Garden........................................................................... 73 4.3.5.2 S-wave first break picking .................................................................................................... 74 4.3.5.3 Time-term inversion and Tomographic inversion in Zen Garden......................................... 76 4.3.6 Comparison with Ground Penetration Radar (GPR) ............................................................... 77 4.3.6.1 GPR (Ground Penetration Radar) ......................................................................................... 77CHAPTER FIVE. ................................................................................................................................. 825.0 RESULTS AND DISCUSSION ............................................................................................... 825.1 Basic refraction analysis in North Line........................................................................................... 82 5.1.1Results from Gradient-Intercept method on the North line ...................................................... 82Junghee Kim 3
  • 5.2 Advanced seismic refraction analysis in North Line ...................................................................... 86 5.2.1. Time Term Inversion .............................................................................................................. 86 5.2.2 Tomographic Inversion ............................................................................................................ 89 5.2.3 Reciprocal Method ................................................................................................................... 995.3 Statics analysis of P-wave data in North Line .............................................................................. 103 5.5.1 Elevation static correction from first break picks picked in Promax: .................................... 103 5.5.2 Datum statics from tomographic inversion . .......................................................................... 104 5.5.3 Application of static correction to the stack ........................................................................... 106 5.5.4 Comparison of the stack with results from refraction analysis. ............................................. 109 ........................................................................................................................................................ 111 5.5.5 Comparison with the result of DC-resistivity survey in North line area. ............................... 1125.4 Advanced refraction analysis in Zen Garden ................................................................................ 114 5.4.1 P-wave velocity model analysis in Zen Garden ..................................................................... 114 5.4.2 S-wave Velocity model from tomographic inversion in Zen Garden .................................... 119 5.4.3 Poison’s ratio analysis............................................................................................................ 121 5.4.4 Vp/Vs analysis ....................................................................................................................... 123CHAPTER SIX. .................................................................................................................................. 1256.0 Conclusions and Recommendations ....................................................................................... 125References ........................................................................................................................................... 127Appendix ............................................................................................................................................. 130List of tablesTable 4-1 Summary of data acquisition in Pagosa Springs Colorado USA ........................................................... 42Table 5-1 Depth model from basic refraction analysis ........................................................................................ 85Table 5-2 Velocity model from basic refraction analysis ..................................................................................... 85Table 5-3 Seismic Velocities of Earth Materials (Gary Mavko, 2005) .................................................................. 99Table 5-4 P- to S-wave velocity and Poisson’s ratios calculated from P- and S-wave in Zen Garden ................ 121Junghee Kim 4
  • List of figuresFigure 1-1Seismic waves and the behaviour at interfaces .................................................................................... 10Figure 2-1 Location of Pagosa Springs in entire map of United States of America. (Map is copyright GoogleEarth) .................................................................................................................................................................... 13Figure 2-2 Areal Map of the Structures in the San Juan Basin with the area of Pagosa Springs outlined in red (Imperial College London and Colorado School of Mines Students of the geophysics field camp, 2012) .............. 13Figure 2-3 Areal map with the Archuleta anticlinorium showing relations with the San Juan Basin and otherbasin. ( Imperial College London and Colorado School of Mines Students of the geophysics field camp, 2012) .. 14Figure 3-1 Relationship between the angles of incidence and refraction ............................................................. 15Figure 3-2 Source-to-receiver raypath of a refracted ray in a two-layer case. ..................................................... 16Figure 3-3 Traveltime-offset curve for a horizontal interface two-layer case ...................................................... 17Figure 3-4 Source-to-receiver raypath of a refracted ray in a three-layer horizontal case ................................... 18Figure 3-5 Traveltime-offset curve for a horizontal interface three-layer case .................................................... 20Figure 3-6 Hidden layer problem in refraction caused by a layer having insufficient thickness and velocitycontrast................................................................................................................................................................. 21Figure 3-7 Blind layer problem in refraction caused mainly by a velocity inversion. ............................................ 22Figure 3-8 Refraction picking options: t0 is the first break (first kick) time, t1 is the first arrival time through thefirst inflection time, and t2 to t7 are the trough, zero crossing, and peak times (Cox, 1999) .............................. 23Figure 3-9 Principle of the delay-time method ..................................................................................................... 25Figure 3-10 Principle of reciprocal method ........................................................................................................... 26Figure 3-11 Principle of reduced traveltimes ........................................................................................................ 28Figure 3-12 Principle of time-term inversion (in case that the refractor is parallel to the ground surface) ......... 31Figure 3-13 Principle of time-term inversion (in case that the refractor is non-parallel to the ground surface) .. 33Figure 3-14 Process of depth calculation in time-term inversion.......................................................................... 34Figure 3-15 Principle of tomographic inversion .................................................................................................... 35Figure 4-1 Project work-flow ................................................................................................................................ 39Figure 4-2 Data Acquisition work-flow ................................................................................................................. 40Figure 4-3 hammer seismic showing different p-wave ray paths ......................................................................... 41Figure 4-4 Data acquisitions of P-wave and S-wave ............................................................................................. 41Figure 4-5 Elevation profile of survey area (North line) (information from GPS in Colorado field camp) ............. 43Figure 4-6 Data conversion work-flow .................................................................................................................. 43Figure 4-7 General Cross-section of Pagosa Springs showing location of North line and Zen Garden withexaggerated vertical scale in larger detail. ........................................................................................................... 44Figure 4-8 map of survey area (Map is copyright Google Earth) ......................................................................... 45Figure 4-9 work-flow of basic refraction analysis in North Line........................................................................... 46Figure 4-10 Geometry assignment screen of Common Depth Point (CDP) and Fold in Promax ........................... 47Figure 4-11 Deciding what pick to make for the first arrivals, First Kick, Trough or Peak. ................................... 48Figure 4-12 First break picking on first-kick in Promax ......................................................................................... 48Figure 4-13 Gradient-intercept method graph ..................................................................................................... 49Figure 4-14 work-flow of advanced refraction analysis in North Line .................................................................. 50Figure 4-15 Original data before applying any form of gain. ............................................................................... 51Figure 4-16 Data in figure 4-15 after amplitude correction, stretching. .............................................................. 51Figure 4-17 Reciprocal test for two shots with significant refractor overlap. ....................................................... 52Figure 4-18 Example of P-wave first break picking on first-kick ........................................................................... 53Figure 4-19 Example of layer assignment ............................................................................................................. 54Junghee Kim 5
  • Figure 4-20 Example of reverse line forming with delay time line for reciprocal method .................................... 55Figure 4-21 Example of Layered model from time-term inversion ....................................................................... 56Figure 4-22 Process of Tomographic inversion ..................................................................................................... 57Figure 4-23 Design of the number of layers for initial model ............................................................................... 58Figure 4-24 Ray tracing path in tomographic inversion ....................................................................................... 59Figure 4-25 work-flow of seismic reflection data processing and analysis in North Line ..................................... 60Figure 4-26 Refraction muting in Promax. (left: before refraction muting, middle: applying refraction muting,right : after refraction muting ) ............................................................................................................................ 61Figure 4-27 Aliased reflectors of data in FK spectrum analysis ............................................................................ 62Figure 4-28 Schematic drawing on cut range of Bandpass (frequency: 50 – 100 - 200 - 400 Hz)......................... 63Figure 4-29 Bandpass filter application ( left: gather before applying bandpass, right: gather after applyingbandpass............................................................................................................................................................... 64Figure 4-30 schematic geometry for elevation statics with data from first break picks on first-kick of Promax .. 65Figure 4-31 schematic geometry for datum statics using data from tomographic inversion of Seisimager ........ 66Figure 4-32 Screen showing difficulties on velocity picking in Promax ................................................................. 68Figure 4-33 Schematic drawing showing possibility of use of constant velocity for Normal Move Out in shortoffset ..................................................................................................................................................................... 69Figure 4-34 Expected reflector through a look into gather in Promax ................................................................. 70Figure 4-35 Reflector shown in Brute stack in Promax ......................................................................................... 70Figure 4-36 work-flow of comparison of North Line with DC-resistivity ............................................................... 71Figure 4-37 SP and inverted resistivity profiles of PAGO 02 (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 72Figure 4-38 North line area where North line hammer seismic survey line crossing with PAGO 02 DC resistivitysurvey line (Map is copyright Google Earth) ......................................................................................................... 72Figure 4-39 Work-flow of advanced refraction analysis in Zen Garden ................................................................ 73Figure 4-40 Example of P-wave firstbreak picking on first-kick in Zen Garden ..................................................... 74Figure 4-41 Example of the raw data of S-wave in Zen Garden ........................................................................... 75Figure 4-42 Example of choosing bad trace of S-wave in Zen Garden .................................................................. 75Figure 4-43 Example of S-wave firstbreak picking on first-kick in Zen Garden ..................................................... 76Figure 4-44 Work-flow of comparison of Zen Garden with GPR ........................................................................... 77Figure 4-45 GPR acquisition comprising of the radar components and the analogue interpretation of a radartime section. Tx: Transmitter, Rx: Receiver (Redrawn from Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 78Figure 4-46 Barn 3 survey line ( red line: SW- NE ) cited from Google Map ........................................................ 79Figure 4-47 General cross-section of Pagosa Springs showing the location of Barn 3 and Zen Garden. Verticalscale has been exaggerated to show features in larger detail. (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 80Figure 4-48 General cross-section of the location of GPR acquisition in data acquisition line of Barn 3. Verticalscale has been exaggerated to show features in larger detail. (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 81Figure 5-1 Depth model generated from picking firstbreak on the first pick in Promax ...................................... 82Figure 5-2 Depth model generated from picking firstbreak on first kick in Promax ............................................ 83Figure 5-3 Depth model generated from picking firstbreak on first trough in Promax ....................................... 83Figure 5-4 Velocity model generated from picking firstbreak on the first pick in Promax ................................... 84Figure 5-5 Velocity model generated from picking firstbreak on first kick in Promax .......................................... 84Figure 5-6 Velocity model generated from picking firstbreak on the first trough in Promax ............................... 85Junghee Kim 6
  • Figure 5-7Connected Layer assignment of whole North line in Plotrefa TM of Seisimage ................................... 87Figure 5-8 Layered model from time-term inversion of North line with smoothing effect (Smoothing passes: 3)added in Plotrefa TM of Seisimager...................................................................................................................... 88Figure 5-9 Principle of designing the number of layers for the initial model ........................................................ 89Figure 5-10 the image of one move-up time term inversion result chosen for parameter tests for initial model inNorth line in comparison with the whole North line time term inversion image in Plotrefa TM of Seisimager ... 91Figure 5-11 images of one pattern time term inversion result chosen for parameter tests for initial model inNorth line in Plotrefa TM of Seisimager ( (a) P-wave velocity 30 m/s – 3000 m/s, the number of layers 10 (b)P-wave velocity 30 m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 3000 m/s, thenumber of layers 18 ............................................................................................................................................. 92Figure 5-12 images of time term inversion result chosen for parameter tests for initial model in North line inPlotrefa TM of Seisimager ((a) P-wave velocity 30 m/s – 1000 m/s, the number of layers 15 (b) P-wavevelocity 30 m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 10000 m/s, the number oflayers 15 ) ............................................................................................................................................................. 93Figure 5-13 The image of initial model in whole North line in Plotrefa TM of Seisimager ( calculated withparameters of P-wave velocity 30 m/s – 3000 m/s, the number of layers 15 ...................................................... 95Figure 5-14 Misfit between synthetic and observed travel time as a function of the iteration number. Observethe lack of significant reduction in the travel time misfit after about 10 iterations. ............................................ 96Figure 5-15 the image of P-wave velocity model from tomographic inversion in whole North line in Plotrefa TMof Seisimager (value 10 was chosen for the number of iteration ) ....................................................................... 97Figure 5-16 the image of Ray tracing path of P-wave velocity model from tomographic inversion in whole Northline in Plotrefa TM of Seisimager .......................................................................................................................... 98Figure 5-17 an image of reciprocal method showing delay time line and reverse time line in one move-up ofNorth line in Plotrefa TM of Seisimager (delay times in both sides are calculated and averaged ) ................... 100Figure 5-18 the image of P-wave velocity model generated by reciprocal method in one move-up of North linein Plotrefa TM of Seisimager ( delay times in both sides are calculated and averaged ) ................................... 101Figure 5-19 Comparison between images of P-wave velocity models generated by reciprocal method and time-term inversion in one move-up of North line in Plotrefa TM of Seisimager (Note that both methods areconducted in same position) ............................................................................................................................... 102Figure 5-20 plots of Elevation static correction on P-wave obtained from first break pick on first kick inNorthline of receiver shown in Promax . ............................................................................................................. 103Figure 5-21 plots of Elevation static correction on P-wave obtained from first break pick on first kick in Northlineof source shown in Promax . ............................................................................................................................... 103Figure 5-22 Values of LVL Static ( refraction static), Elevation static correction of receiver and total datum staticcorrection shown in Promax . The values of elevation static correction and LVL static correction are added up tofind datum static correction. .............................................................................................................................. 104Figure 5-23 plots of Datum static correction on P-wave obtained from tomographic inversion in North line ofreceiver shown in Promax . ................................................................................................................................. 105Figure 5-24 plots of Datum static correction on P-wave obtained from tomographic inversion in North line ofsource shown in Promax . ................................................................................................................................... 105Figure 5-25 the image of stack not applied with static correction (only bandpass applied : Bandpass frequencyrange : 50 – 100 -200 -400 hz . ........................................................................................................................... 106Figure 5-26 the image of stack applied with elevation static correction ( bandpass and elevation staticcorrection applied : Bandpass frequency range : 50 – 100 -200 -400 hz .) ......................................................... 107Figure 5-27 the image of stack applied with Datum static correction ( bandpass and datum static correctionapplied applied : Bandpass frequency range : 50 – 100 -200 -400 hz Here Datum static correction = LVL staticJunghee Kim 7
  • correction ( Refraction static correction (LVL) .................................................................................................... 107Figure 5-28 the image of stack ( only bandpass applied : Bandpass frequency range : 50 – 100 -200 -400 hz . 108Figure 5-29 the image of stack applied with elevation static correction ( bandpass and elevation staticcorrection applied : Bandpass frequency range : 50 – 100 -200 -400 hz .) ......................................................... 108Figure 5-30 the image of stack applied with datum static correction ( bandpass and datum static correctionapplied applied : Bandpass frequency range : 50 – 100 -200 -400 hz Here datum static correction = LVL staticcorrection ( Refraction static correction )+ elevation static correction............................................................... 108Figure 5-31 A possible fault by comparison between refraction processed image and reflection processed imagein North line. (a) image from time-term inversion (b) image from tomographic inversion (c) image from brutestack applied with datum statics correction. ...................................................................................................... 110Figure 5-32 A possible fault (F1) by comparison of the stack with superimposed and flattened refractionprocessed image( from tomographic inversion) in North line ............................................................................ 111Figure 5-33 A possible fault expected by result from DC-resistivity survey and Hammer seismic survey in NorthLine area (The DC-resistivity model is fit to the PAGO02 pararelly, and the tomographic inversion image is fit tothe North line in parallel) DC-resistivity image is cited from Imperial College London and Colorado School ofMines Geophysics Camp 2012. ........................................................................................................................... 113Figure 5-34 the image of P-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TMof Seisimager ...................................................................................................................................................... 114Figure 5-35 the image of P-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration ) ......................................................................... 115Figure 5-36 the image of Ray tracing path of P-wave velocity model from tomographic inversion in Zen Gardenin Plotrefa TM of Seisimager .............................................................................................................................. 116Figure 5-37 Comparison of P-wave velocity model from tomographic inversion and subsurface model from basicgradient intercept method done by Imperial College London and Colorado School of Mines Geophysics FieldCamp, 2012 ( right Figure.- cited from Imperial College London and Colorado School of Mines Geophysics Camp,2012 (right Figure cited from Imperial College London and Colorado School of Mines Geophysics Camp, 2012)............................................................................................................................................................................. 117Figure 5-38 the image of S-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TMof Seisimager ...................................................................................................................................................... 118Figure 5-39 the image of S-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration) .......................................................................... 118Figure 5-40 the image of Ray tracing path of S-wave velocity model from tomographic inversion in Zen Gardenin Plotrefa TM of Seisimager .............................................................................................................................. 119Figure 5-41 Comparison of shapes of P-wave data and S-wave data ................................................................ 120Figure 5-42 Chart of Poisson’s ratio, Vp/Vs ratio and P-wave velocity (Redrawn from Thomas M. Brocher, 2005)............................................................................................................................................................................ 122Figure 5-43 Chart of Vp, Vp/Vs ratio and Porosity in Zen Garden ( Redrawn from E.R.(Ross) Grain, 2000) .... 123Figure 5-44 (a) Seismic section at Barn 3 and (b) its interpretation related to the Dakota Sandstone. (ImperialCollege London and Colorado School of Mines Geophysics Field Camp, 2012) .................................................. 124Junghee Kim 8
  • ACKNOWLEDGEMENTDr. Adam Booth. I would like to express my special appreciation to him. He is my supervisor.Without his guidance and supervision, the completion of this project would not be possible.In addition, I would like to express special gratitude to Professor Helmut for his kind supportsand guidance throughout this entire course.I also appreciate Faculty of Colorado School of Mines for the efforts that are made to acquirethese data from Pagosa Springs, Colorado, USA.Sincere thanks to Mr Seth who was in charge of data acquisition in Pagosa Springs for hiskind support and guidance.Special thanks to My sister, Mrs. In-hee Kim and his husband Mr. Isaac Choi, my parent,Mrs. Sun-hee Kim, Mr. Hyun-dong Kim.And I also thank Kenneth for his spiritual supports.Junghee Kim 9
  • CHAPTER ONE 1.0 IntroductionSeismic surveys measure the earth’s elastic properties using seismic waves (Sheriff 2002).The source of these disturbances can be controlled as in the case of exploration andengineering seismology, or it can be uncontrolled as in the case of earthquake seismology.(Dobrin 1976) The propagation is described by the elastic wave equation, which is derivedfrom two laws of physics, Hooke’s law and Newton’s second law of motion. (Dobrin 1976)When an elastic wave propagates through a medium in the earth is reflected, refracted andtransmitted at an interface (Figure 1-1) (Dobrin 1976). The wave can also be diffractedaround discontinuities. (Dobrin 1976) Figure 1-1 Seismic waves and the behaviour at interfaces (Dobrin 1976; Waters 1997)There are two forms of seismology, reflection and refraction seismology (Jakubowicz 2012).Refraction seismology involves the recording, processing and analysis of refracted seismicenergy and is mainly used for near surface studies. Reflection seismology involvesprocessing and analysing seismic reflected energy. Reflection surveys are mainly applied inexploration for mining and hydrocarbon exploration (Dobrin 1976), and crustal studies(Reading et al, 2011). Seismic experiments performed for near surface investigations arereferred as shallow seismic surveys. (Karastathis et al. 2007)Shallow seismic studies are often applied to detect geologic structures in fault zones and tofind shallow, soft layers of underground earth materials especially in area of rapidJunghee Kim 10
  • urbanisation and heavy agriculture. (Karastathis et al. 2007)Seismic refraction survey using a Hammer source was conducted along selected line acrossPagosa Springs, Colorado in June 2012. The aim was to perform near surface study andcharacterisation of the hydrothermal activities in the area. Although Pagosa Springs inColorado is famous for the hydrothermal activities, these are still not well understood.(Imperial College London and Colorado School of Mines Geophysics Field Camp 2012)In this project, near surface study and characterisation using refraction analysis of dataacquired at Pagosa Springs will be performed with a view to determining the depth of thebedrock and the ground water, the lateral and vertical changes in lithology, the lithology typeand investigating the structural features such as micro faults.1.1 ObjectivesThe aims of the near surface study in Pagosa Springs are as follows:  To use P-wave and S-wave refraction methods to obtain velocity-depth models for near-surface layering at Pagosa Springs.  To combine P- and S-wave observations to quantify physical properties of near- surface layering, and to propose lithology.  To investigate the interpretation of P-wave refraction data as a reflection profile, including a near-surfaceJunghee Kim 11
  • CHAPTER TWO 2.0 Geological setting of Pagosa Springs, Colorado USAPagosa Springs is located on the northeast edge of the San Juan Basin as seen in Figure 2-2. ( Imperial College London and Colorado School of Mines, geophysics filed camp 2012)This is a large depositional basin concentrated in western New Mexico and Four Cornersregion of the western United States (Fred 1982).The basin is bordered in the north by theSan Juan Mountains of southern Colorado, in the northeast by the Chama Basin, in the eastby the Nacimiento and San Pedro Uplifts, in the south by the Zuni Uplift and the ZuniMountains of New Mexico and in the west by the Defiance Uplift of eastern Arizona andwestern New Mexico. The central basin with deepest sedimentary units is mainly located innorth western New Mexico and a small part of southern Colorado. (Fred 1982) Uplift ofmountain ranges almost prior to the Cambrian age and the transgression of multipleseaways beginning in the late Cambrian age caused this basin to form. This is the reasonwhy the basin includes almost continuous column of sedimentary units beginning in the lateCambrian and continuing until the glaciations and orogenies of the late Cenezoic. (Fred1982). On the Archuleta anticlinorium, Pagosa Springs is located in the northeast edge ofthis basin. (Fred 1982) The Archuleta anticlinorium is located in the edge of the San JuanBasin starting from southern Colorado with a north- northwest trend, continuing into northcentral Arizona. (Fred 1982) The region is located 15 miles west of the continental dividewith the San Juan River serving as the primary stream system because it flows from theDivide to the Pacific Ocean to the Southwest. Its allochthonous folding over the underlyingbasement is the most significant characteristics of this structure. (Fred 1982) A shallownorth-north western trending anticline through Pagosa Springs is produced by this. Thisgives the 12000 ft of sedimentary units in the area, a dip of about 5-10˚ towards the SanJuan Mountains in the north eastern half of the anticlinorium and a similar dip towards thebasin on the south western half. (Fred 1982) To the north, the units merge with thesurrounding basins beneath the San Juan Mountains. (Fred 1982) However, to the south,the units increase in dip when they move towards the main basin. (Fred 1982) In the Pagosa Springs, Colorado USA, geophysical analyses have been performed bystudents of Imperial College London and Colorado School of Mines during the geophysicalsummer camp 2012. Different geophysical experiments were performed in this area. One ofsuch was the refraction seismic method which is to analyse near surface geology of the areafor geotechnical and groundwater purposes.Junghee Kim 12
  • Figure 2-1 Location of Pagosa Springs in entire map of United States of America. (Map is copyright Google Earth) Figure 2-2 Areal Map of the Structures in the San Juan Basin with the area of Pagosa Springs outlined in red ( Imperial College London and Colorado School of Mines geophysics field camp 2012)Junghee Kim 13
  • Figure 2-3Areal map with the Archuleta anticlinorium showing relations with the San Juan Basin and other basin. (Imperial College London and Colorado School of Mines, geophysics field camp 2012)Junghee Kim 14
  • CHAPTER THREE 3.0 Theory and Literature review3.1 Refraction Seismic MethodRefraction can be defined in terms of the change in direction of a seismic ray or wavefront atan interface between layers of different velocities (Cox 1999). The relationship between theangles of incidence and refraction at the interface (Figure 3-1) is governed by Snell’s law,which is given as (Craig Lippus 2007): (2.1)Where , represent the angles of incidence and refraction and , represent thevelocities in the first and second layer respectively. (Craig Lippus 2007) Figure 3-1Relationship between the angles of incidence and refraction (Jacob Fokkema and Nafi Toksoz 2012)When the angle of incidence is such that the refracted wavefront is perpendicular to theinterface ( ), it is referred to as critical angle of incidence ( ) and the refracted ray travelsalong the interface between the two layers. Equation (2.1) is the then adjusted to the form(Craig Lippus 2007):: (2.2)Junghee Kim 15
  • The waves that travel to and along the interface between the two layers and return to thesurface through the upper layer are referred to as refraction waves, head waves, Mintropwaves, or bow waves (Cox 1999).3.2 Time-Distance curves for layered mediaFigure 2.5 shows the raypath of a refracted ray from a source location at S to a receiverlocation at R for a two-layer horizontal interface case. The total traveltime ( ) for thisraypath, having a source-to-receiver separation of x is given as the sum of the traveltime oneach of the three sections making up the path. (Jacob Fokkema and Nafi Toksoz 2012) i.e: (2.3)This implies that:Rearranging the equation: (2.4)Figure 3-2 Source-to-receiver raypath of a refracted ray in a two-layer case (Jacob Fokkema and Nafi Toksoz 2012).Using Snell’s law (Jacob Fokkema and Nafi Toksoz 2012)Junghee Kim 16
  • (2.5)Finally we have: (2.6)Equation (2.5) represents a straight line with a slope of and an intercept of given by: (2.7)Figure 2.5 shows the traveltime graph representing the propagation of the refracted ray for atwo-layer horizontal case. From the graph we can calculate and use it to estimate to therefractor z. (Jacob Fokkema and Nafi Toksoz 2012)Figure 3-3Traveltime-offset curve for a horizontal interface two-layer case (Jacob Fokkema and Nafi Toksoz 2012)From Equation (2.7), we have that (Jacob Fokkema and Nafi Toksoz 2012):Junghee Kim 17
  • (2.8)Using equation 2.2 and some trigonometric properties, we have that (Jacob Fokkema andNafi Toksoz 2012) : (2.9)Figure 3-4 Source-to-receiver raypath of a refracted ray in a three-layer horizontal case (Jacob Fokkema and NafiToksoz 2012)For a three-layer case having a raypath diagram shown in figure 3-4, Equations (2.5 – 2.7)can be extended following the same processes as above to yield the total traveltime asJunghee Kim 18
  • (Jacob Fokkema and Nafi Toksoz 2012), (2.10)This again is a straight line equation with a slope of and an intercept of given as: (2.11)The depth of the first layer is calculated as before, while the thickness of the second layer isgiven as: (2.12)Therefore, (2.13)Junghee Kim 19
  • Figure 3-5 Traveltime-offset curve for a horizontal interface three-layer case (Jacob Fokkema and Nafi Toksoz 2012)Figure 3-5 shows the traveltime curve for the three layer case from which we read theintercept times and calculate the thicknesses of the various interfaces.For a multilayer problem, Equation (2.14) is given by (Cox 2009) (2.14)Where (2.15)3.3 Hidden Layers, Velocity Inversions, and Blind ZonesIn order to be detected in a first arrival refraction survey, a layer must satisfy two conditions:(a) be underlain by a layer of higher velocity so that head waves are produced, and (b) havea thickness and velocity such that the head waves become first arrivals at some range(Kearey and Brooks, 2002). It is possible for layers to exist in the Earth, yet not produce anyrefracted first-arrival waves, and a simple first arrival refraction survey will not be able toJunghee Kim 20
  • detect these layers if these conditions are not met. The possibility of undetected layersshould therefore be considered when interpreting refraction data. (Philip Kearey et al. 2002)Figure 3-6 Hidden layer problem in refraction caused by a layer having insufficient thickness and velocity contrast(Philip Kearey et al. 2002).In practice, two different types of problem are shown: (1) Hidden layer, and (2) Blind zone.A hidden layer, from its name, is one that cannot be detected by first arrival seismicrefraction method, and may be caused by insufficient thickness and velocity contrast of thelayer (Cox, 1999). The layer produces head waves, but does not give rise to first arrivals(Kearey and Brooks, 2002). Rays travelling to deeper levels arrive before those criticallyrefracted at the top of the layer in question (Figure 3-6). In such a case, a method of surveyinvolving recognition of only first arrivals will fail to detect the layer. It is good practice toexamine the seismic traces for possible arrivals occurring behind the first arrivals. (PhilipKearey et al. 2002)A blind layer violates the first condition necessary for first arrival refraction experimentdetection by resulting from a low-velocity layer, as illustrated in Figure 3-7 (Kearey andBrooks 2002). Rays are critically refracted at the top of such a layer and the layer willtherefore not give rise to head waves. The interpretation of travel-time curves, in thepresence of a low-velocity layer, leads to an overestimation of the depth to underlyinginterfaces. (Philip Kearey et al. 2002)Junghee Kim 21
  • Figure 3-7 Blind layer problem in refraction caused mainly by a velocity inversion (Philip Kearey et al. 2002).3.4 Refraction Arrival picking and time adjustmentsThe first step in the interpretation of a refraction experiment data is to review and pick thearrival times (Cox 1999). While the review phase involves the initially analysis of the data tobe picked, the picking phase is concerned with the actual picking of traveltimes, which isusually done either manually or automatically. Certain adjustments of reciprocal time arealso performed on the picked traveltimes before any form of interpretation is then carried out.(Cox 1999)3.5 Manual picking and automatic picking of traveltimesFigure 2.10 shows a refraction arrival in which the various forms of picks (from first kick,peak, trough) has been shown. Picking requires that we have a broadband signal, minimalfiltering of data, a good signal-to-noise ratio, and a high gain display (Cox 1999). First breakor kick (represented by t0 in Figure 3-8 ) is usually picked because a change in frequencywith offsets, receiver and source locations (usually common with land surveys) may cause ashift relative to the first break. (Cox 1999)Junghee Kim 22
  • Figure 3-8 Refraction picking options: t0 is the first break (first kick) time, t1 is the first arrival time through the firstinflection time, and t2 to t7 are the trough, zero crossing, and peak times (Cox 1999)In most settings, it is desirable in manual picking of travels times that the accuracy stayswithin 1 or 2 ms for individual picks (Cox 1999).In the presence of a large dataset the picking is usually automated. Automated picking workswell in a good signal-to-noise dataset, and the first arrivals are well defined. (Cox 1999)3.6 Reciprocal Time CorrelationRegardless of the subsurface structure, seismic reciprocity condition between any two pointsmust be satisfied for the surface-consistent refracted travel times,(Hagedoorn 2006) i.e.: (2.16)This condition should be tested and corrected prior to performing any form of interpretation.It is usually done by calculating the reciprocal time misfits between all pairs of shot locations(Si and Sj) with reciprocal (reversed) recording (Hagedoorn 2006): (2.17)When the misfit ( ) is large, corrections are then applied to traveltime picks, though it isadvised that the picking be redone when possible (Hagedoorn 2006).Junghee Kim 23
  • 3.7 Refraction InterpretationIn an area with simple planar refractors and the velocities in the overlying layers are laterallyinvariant, any of Equations (2.4) to (2.17) can be used to determine the layer velocities andtheir corresponding depths. However, in practice the geology is usually very complex andspecial efforts are therefore required in refining these equations and in applying themsubsequently (Jacob Fokkema and Nafi Toksoz 2012).Refraction interpretation methods are broadly divided into two approaches (Cox 1999):Those in which the data are analysed at a common surface location and those in which thedata are analysed at a common subsurface location.Inversion can also be used to interpret refraction data. Tomographic and time-terminversions are the most common applied in practice.3.8 Gradient-Intercept methodThe gradient-intercept method (also called intercept method) is used as an interpretationmethod when the geology is simple and planar. It uses the Equations derived above ((2.4) ~2.17)), where the intercept time (zero offset time) is used to determine the refractor depth atthe source location (Jacob Fokkema and Nafi Toksoz 2012). (Figure 3-2).3.9 Delay-Time ConceptIn a complex subsurface where the interfaces are undulating and multi-layered, most of therefraction-statics methods, such as the Plus-Minus and the Generalized Reciprocal methodsare based on the delay-time approximation of refracted travel times (Hagedoorn 2006) tosolve for the refraction statics. Consider a source located at point S and a receiver at pointR at the surface (Figure 2.4). In the delay-time approximation, the refractor is considered asnear-horizontal between the two points, and the distance between them is much greater thanthe critical distance. (here, critical distance means the minimum distance from the energysource at which the first critical refraction can be received (Jacob T. Fokkema and M.NafiToksoz 2012). Generally, this implies that the velocity of the refractor (bedrock) is muchlarger than that of the overburden.Under these approximations, the travel-time from S to R can then be separated to thesource-side and receiver-side times (Jacob Fokkema and Nafi Toksoz 2012).: (2.18)Junghee Kim 24
  • Figure 3-9 Principle of the delay-time method (Jacob Fokkema and Nafi Toksoz 2012).Time can be represented as a sum of the travel time along the reflector and the “sourcedelay” time (Jacob Fokkema and Nafi Toksoz 2012).: (2.19)For source delay, , we therefore have (Jacob Fokkema and Nafi Toksoz 2012): (2.20)In a similar way, the receiver delay time is defined, and the total time from the source to thereceiver is (Jacob Fokkema and Nafi Toksoz 2012) : (2.21)This equation relates the velocity of the bedrock and the depth of the weathering layer to thefirst-arrival travel times. This equation is further inverted to solve for the depths of theweathering layer near the sources and receivers, and the velocity of the refractor (JacobFokkema and Nafi Toksoz 2012).Junghee Kim 25
  • 3.10 Reciprocal MethodConcept of Delay time in Reciprocal Method is as Figure 3-10. Figure 3-10 Principle of reciprocal method (Jacob Fokkema and Nafi Toksoz 2012).Referring Equation (2.19) and Equation (2.20), if AC = BD, in this case, × 2 (becausein both sides) + (here x = ) (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2DManual 2005).. (2.22)But if it is different values, Then, (2.23) Similarly, (2.24)AndJunghee Kim 26
  • (2.25)Delay time to in Reciprocal method (2.26)If substituting, (2.27) This is equal to, (2.28)In the Figure 3-10, (2.29)Junghee Kim 27
  • Therefore, (2.30)Here, to is twice the time required for the seismic energy to travel from P to P’.Delay time DT at point P is defined as below (Jacob Fokkema and Nafi Toksoz 2012;Seisimager/2D Manual 2005).. . (2.31)Computation of reduced traveltime allows us to remove the effect of changing layerthickness on the traveltim curve and give a better measurement of velocity. The delay timeand refractor depth are calculated (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2DManual 2005).. . Figure 3-11 Principle of reduced traveltime (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2D Manual 2005)The reduced traveltime at point P for a source at A T’AP (Jacob Fokkema and Nafi Toksoz2012; Seisimager/2D Manual 2005)..Junghee Kim 28
  • (2.32)This is same as (2.33)By rearranging, (2.34)Because (2.35) (2.36)Therefore, (2.37)Assuming that the AC = BD, (2.38)Junghee Kim 29
  • (2.39)Because (2.40)Therefore, (2.41) (2.42)Therefore, the depth in P point is decided as following (Jacob Fokkema and Nafi Toksoz2012; Seisimager/2D Manual 2005).. (2.43)Note that Equation (2.43) is same as (Jacob Fokkema and Nafi Toksoz 2012;Seisimager/2D Manual 2005). (2.44)Junghee Kim 30
  • 3.11 Term-time inversionA linear Least-Squares approach is used to define the time-term method. This is todetermine the best discrete-layer solution to the data (Takaya Iwasaki 2002; Seisimager/2DManual 2005).Figure 3-12 Principle of time-term inversion (in case that the refractor is parallel to the ground surface) (TakayaIwasaki, 2002; Seisimager/2D Manual 2005). .Slowness is defined as S which is inverse velocity (Takaya Iwasaki 2002; Seisimager/2DManual 2005). . (2.45) (2.46)Junghee Kim 31
  • From Snell’s Law, (2.47)Travel time definition in reciprocal method (in the assumption that the depths in both sidesare same) (2.48)If the total travel time = t from source to receiver, h = z, S1 = 1/V1, S2 = 1/V2 (2.49)C is defined as follows, (2.50)Then (2.51)Z and S2 are not known The example above has assumption that the refractor is parallel to the ground surfaceIf these are non-parallel, curved surfaces, there are three un-knowns Z1, Z3 and S2. (TakayaIwasaki 2002; Seisimager/2D Manual 2005). .Junghee Kim 32
  • Figure 3-13 Principle of time-term inversion (in case that the refractor is non-parallel to the ground surface) (TakayaIwasaki 2002; Seisimager/2D Manual 2005). .Now, (2.52)Generalisation, (2.53)In matrix form,Junghee Kim 33
  • (2.54)Where m = number of traveltimes, and n = number of receivers (Depths to be calculated).So, Z1, Z2, ••• Zn and S2 are solved. Figure 3-14 Process of depth calculation in time-term inversionTo make it clear, in Figure 3-14, the first source can have many cases of x values withdifferent t values. When the seismic ray is passing P1, many receivers can receive this ray.By the travel times and x values, z1 is decided. The second source does same thing againcalculating z2 and it is repeated up to the last source calculating z3, z4, ··· zn. This is ··,expressed as Equation (2.54).Junghee Kim 34
  • 3.12 Tomographic inversion methodJacob R. Sheehan et al. (2000) stated that tomographic inversion method is able to resolvevelocity gradients and lateral velocity changes and can be applied in settings whereconventional refraction techniques don’t work. For example, the method can be applied inareas of compaction, karst, and fault zones.Tomographic inversion requires an initial model because this inversion is non-linear problem.Iteratively tracing rays through the model compares the calculated traveltimes to themeasured traveltimes. And it modifies the model and repeats the process until the misfitbetween calculated and measured times is minimised. Therefore, the ultimate goal is to findthe minimum traveltime source and receiver for each source-receiver pair. By solving l(raypath) and s (slowness: inverse velocity). Because both are unknowns, the problem isunder-constrained and an iterative, least-squares approach. (Non-linear problem) (Jacob R.Sheehan et al. 2000 ; Seisimager/2D Manual 2005). Figure 3-15 Principle of tomographic inversion (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005). (2.55)S= slowness = velocitylij = raypathJunghee Kim 35
  • (2.56)Therefore, (2.57)Or (2.58)Following can be said. ● (2.59) ●This can be expressed asJunghee Kim 36
  • (2.60)This is the Least squares method. Generally, M > NThe conditions are required in the tomographic inversion.First, Jacobian matrix requires ray-path.Second, Ray-path cannot be calculated without a velocity model.Third, cannot solve at once.Fourth, must use non-linear Least Square method.Iterative solution of a non-linear Least Squares matrix is as follows. 1) Theoretical value Yo (travel time) for initial value Xo (Slowness) is calculated. (2.61) 2) Calculate residuals (∆Y) between theoretical value Yo and observed value Y. (2.62) 3) Calculate correction value for X(∆Y) by the least squares method (Here, A = raypath) (2.63) 4) Calculate new estimate for X1 ( there X1 = Xo + ∆X ) 5) Put the X1 value back to the model. (2.64)Junghee Kim 37
  • This process is repeated until the misfit is close to the minimum.And with the X values (Slowness) and Y values (travel time), the depths of each point aredecided. (Jacob R. Sheehan et al. 2000; Seisimager/2D Manual 2005)In the time-inversion and tomographic inversion, RMS error checking was performed for dataquality purpose.Here Root-mean-square error (2.65)Here n is the number of layer, and Ei is the difference between the inverted and actualvelocities for the ith layer. (Khaled Al Dulaijan 2008)Junghee Kim 38
  • CHAPTER FOUR 4.0 METHODOLOGYThis section introduces the source of data acquisition, its preparation technique, dataprocessing and methods of analyses. Procedure of this project is as follows in Figure 4-1. Figure 4-1Project work-flowJunghee Kim 39
  • 4.1 Data acquisition Figure 4-2 Data Acquisition work-flowThe location of the North line in the Pagosa Springs, 2012 firstly was chosen for survey isbecause according to geological study, this area is assumed to have anomalous featuressuch as fault, and dipping interfaces.(Imperial College London and Colorado School ofMines Geophysics Field Camp 2012) On the location map of the North Line, P-wave seismicrefraction acquisition was performed. Secondly, the location of the Zen Garden was chosenfor survey because this area is very close to North line, the geological feature in this area isassumed to be similar to the North line area. In addition, in the Zen Garden area, S-waveseismic refraction acquisition, as well as P-wave seismic refraction acquisition has beenperformed. The availability of S-wave and P-wave information allow us to calculate Poisson’sratio and Vp/Vs through which the rock properties, lithology, porosity and water spreading inthe area could be analysed.In North Line, shot and receiver spacing were each 3 m, while the shot point was in sameposition of receiver point. In Zen Garden, shot and receiver spacing were each 3m, while theshot point was midway between two adjacent receivers and 24 geophones were deployed ata time in each line making the maximum offset 70.5m. In Gen Garden the shot moves inbetween the geophone spread, down to the end of the line resulting in a total of 24 shots.In North Line, the shot moves in same position of geophone spread, down to the end of theline resulting in a total of 24 shots. Then the setup is rolled along the line until the end of thesurvey line is reached. The experiment was rolled seven times on the North Line, but donejust once on the Zen garden line.P-waves were recorded in both the North line and the Zen garden using vertical geophones,while an addition S-wave survey was carried out in the Zen garden, using horizontalgeophones (Figure 4-4).Junghee Kim 40
  • Figure 4-3 hammer seismic showing different p-wave ray paths Figure 4-4 Data acquisitions of P-wave and S-waveA summary of the acquisition set is shown in table 4-1.Junghee Kim 41
  • Table 4-1 Summary of data acquisition in Pagosa Springs Colorado USA(Imperial College London and ColoradoSchool of Mines Geophysics Field Camp 2012)Zen Garden area is almost flat (elevation: about 2141 m) and the North line area hastopography as shown in Figure 4.5. (Imperial College London and Colorado School of MinesGeophysics Field Camp 2012) (Appendix. 6)Junghee Kim 42
  • Figure 4-5 Elevation profile of survey area (North line) (information from GPS in Colorado field camp) 4.2 Data conversion Figure 4-6 Data conversion work-flowWhen the data were acquired, the file format was SU file. To process the data, the SUformat file had to be converted to SEG-Y file and SEG-2 file.Matlab was used to convert SU format files to SEG-Y for application in Promax for basicanalysis and reflection processing and SEG-2 format files for application in Seisimager foradvanced analysis. ( Mathworks 2012)Promax and Seisimager will be explained later.Junghee Kim 43
  • Figure 4-7 General Cross-section of Pagosa Springs showing location of North line and Zen Garden with exaggerated vertical scale in larger detail. ( Imperial College London andColorado School of Mines Geophysics Field Camp 2012 )In Figure 4-7, the blue arrow is directing the locations of North line and Zen Garden. (Imperial College London and Colorado School of MinesGeophysics Field Camp 2012)Junghee Kim 44
  • Figure 4-8 map of survey area (Map is copyright Google Earth)Junghee Kim 45
  • 4.3 Refraction Data AnalysisRefraction analysis basically involves the processing and interpretation of first for variousnear surface parameter estimation.4.3.1 Basic refraction analysis in North Line Figure 4-9 work-flow of basic refraction analysis in North Line 4.3.1.1 PromaxSEG-Y format file is used for this process. With the hammer seismic data in Promax,process of the first break picking is conducted.(Promax 1998) Based on the data obtainedfrom this process, Seismic refraction analysis has been performed further in matlab for thebasic analysis. 4.3.1.2 Geometry assignmentIn this process, geometry information of shot spacing (3 m), receiver spacing (3 m) andmove-ups (patterns)(1 – 24, 25-48, 49 -72, 73 -96, 97 -120, 121 – 144, 145 -168)) have beenassigned.Junghee Kim 46
  • Figure 4-10 Geometry assignment screen of Common Depth Point (CDP) and Fold in PromaxThe acquisition was done every move-up (pattern) separately. Once it was done, the linewas rolled up and spread out another line of another pattern. We repeated the process 7times. That is why the fold versus CDP graph looks as 7 peaks.Maximum fold of coverage in Land data (North Line) = The number of channels / (shotinterval/group interval) = 24 / (3/3) = 24 (Jakubowicz 2012) 4.3.1.3 Initial data analysis and quality controlThe original seismic data are initially subjected to quality in other to look for bad shotgathers. The following shot gather were discovered to be really and as such not suitable foranalysis and interpretation. In the initial stage, data were quality controlled for repeatedshots. They were subsequently removed from the dataset. (Appendix 5) 4.3.1.4 First Break Picking in PromaxFirst break picking is to detect or pick the onset arrivals of refracted signals from all thesignals received by the receiver and produced by a source generated. This is sometimescalled first break detection or first arrival picking. (Chugn-Kuang and Chu and Jerry Mendel1994) In this project, first break picking has been done using Promax in each shot.Picking first arrival is faced with the decision of what to pick, First Kick, Peak, or Trough(Figure 3-9).Junghee Kim 47
  • Figure 4-11 Deciding what pick to make for the first arrivals, First Kick, Trough or Peak.Picks were made in this project by selecting first kicks first, peak and later trough.In this project, to see the sensitivity by first break picking, first-kick, peak and trough of theseismic have been picked and the results (Depth models and Velocity models) from thedifferent first-break picks have been compared. Figure 4-12 First break picking on first-kick in Promax 4.3.1.5 Extraction to MatlabThe data of first break picks were extracted and loaded to Matlab for refraction analyses(basic analysis: gradient -intercept method).Junghee Kim 48
  • 4.3.1.6 Gradient intercept methodThe gradient intercept method discussed in chapter was first used to interpret the pickedtravel times. Because the travel time picks do not fall on a straight line, a line of best fit so-called polyfit was used to approximate a straight line representing the picks in MATLAB(Figure 4-13). The test of error between actual data and data from polyfit are measured inAppendix 7. Figure 4-13 Gradient-intercept method graphThe velocities of the first and second layers (and third layers in some case) are estimatedfrom the slopes of each segment of the plot. The thickness of each layer is also estimatedusing the intercept formulae derived in chapter 3. These velocity and thickness values areplaced at the source position and interpolated with the other values at every source position.The results will be in Chapter 5.Junghee Kim 49
  • 4.3.2 Advanced refraction analysis (North Line) Figure 4-14 work-flow of advanced refraction analysis in North Line 4.4.2.1 SeisimagerSEG-2 format file is used for this process. Seisimager has two main modules. PickwinTMand PlotrefaTM. PickwinTM helps to conduct first break picking and PlotrefaTM helps toanalyse the data. Seisimager is a tool for refraction analysis. (Seisimager Manual, 2005). Inthis project, the Seisimager has been used. 4.4.2.2 Initial data analysis and quality controlThe data loaded in Seisimager are checked and bad data are removed. The removed datawere equal to the data removed in Promax. Some data in Zen Garden especially S-wavedata had a lot of noise. Some trace did not have any information. Some traces were killed insome cases and some traces were not applied with first break picks by skipping picking inthe trace. Bandpass was considered. However, by concluding the data given are ok withfirst break picking because it can still showing the first break picks even though it is a lotnoisy deep down. 4.4.2.3 Data ProcessingThe data are uploaded to computer and Seisimager processes the seismic data. Usingfunction of PickwinTM, the first arrival times are picked. (Seisimager 2005)Complete analysis process is as following steps. (Anne Obermann 2000) 4.4.2.4 Elevation importing.The elevation data were imported to the Seisimager before processing for the North linewhile for the Zen Garden, the area is regarded as flat area. The elevation was set as 2141 min Zen Garden. .Junghee Kim 50
  • 4.4.2.5 Amplitude RecoveryThe refraction data may have suffered from amplitude decay due to spherical divergenceand other factors. It is also possible that there have one or two dispersion phenomena in thedata. It is therefore, necessary that before making any pick on the data, some form ofconditioning (which includes amplitude recovery) should be made on the refraction data. Figure 4-15 Original data before applying any form of gain.Figure 4-15 shows the original data as acquired, without any kind of processing applied to it.Obviously, picking on a dataset as this is not practical. The dataset is therefore corrected foramplitude decay, stretched so as to display a few initial times, as we have no need for latearrivals, and finally the amplitudes are clipped to avoid errors in the auto-picker. Figure 4-16shows the corrected form of the same data as figure 4-15. First arrivals picking can now bedone on some data as Figure 4-16. Figure 4-16 Data in figure 4-15 after amplitude correction, stretching.Junghee Kim 51
  • 4.4.2.6 Travel Time Pick and QCHaving corrected for amplitude, first arrivals are then picked and interpreted. 4.4.2.7 Reciprocal Time CheckA basic principle of refraction seismic method is that time reciprocity is valid, i.e.interchanging the source and the receiver positions does not change the arrival time of therefraction events (Phillip Kearey et al. 2002).The error in the reciprocal time is therefore used a QC test for the quality of picks made.Errors greater than 5% of the traveltime suggests that the pick was bad and as such shouldbe repeated. Figure 4-17 shows a sample of a reciprocal time test made in this project.Clearly the error is minimal and hence suggests that this pick is very good. The test isperformed for the entire line using sets of shots having significant refractoroverlap.(Appendix 8.) Figure 4-17 Reciprocal test for two shots with significant refractor overlap.Junghee Kim 52
  • 4.4.2.8 First break picks of P-wave in North Line Figure 4-1 Example of P-wave first break picking on first-kickThe whole 7 move-ups have been first break picked and each move-up has been first breakpicked individually. The first break picks of whole 7 move-ups are to show the whole seismicrefraction map and the individual first break picks are for showing individual seismicrefraction image of interesting area. At this time, the first break picks were picked at first kickpoints (Note that the hammer seismic source is impulsive energy which is minimum phase.So, first break picks would be the first energy that is detected.). The first break picks havebeen picked every 3 shot. 4.4.2.9 Advanced Seismic Refraction Analysis using SeisimagerThe travel times picked are interpreted using gradient, reciprocal method (a betterinterpretation method with no assumption of plane interface), Inversions techniques (Timeterm and tomographic). 4.4.2.10 Layer assignmentThe seismic refraction methods such as reciprocal method, time-term inversion are usingthe concept of delay time as discussed in chapter two. The processing software used(Seisimager PlotRefra) relies on the user to assign layers on the travel time picks. Figure 4-19 shows the layer assignment done for one example. It is worth noting that great care hadbeen taken in picking the travel times as the affect the results of any interpretation algorithmstrongly.Junghee Kim 53
  • Figure 4-19 Example of layer assignment 4.4.2.11 Reciprocal methodAccording to Jocelyn Dufour and Darren Foltinek (2000), the reciprocal method (in otherwords, delay time method) is developed to solve the time delays of reflection seismic data.Based on the determination of the crossover point and reciprocity, the method is performed.In this project, area of West to East distance 85 m to 144 m in North Line has been chosenfor this analysis since this method can analyse only reciprocal time window area whichshould be chosen. The result is compared with result from the other methods in the NorthLine.Junghee Kim 54
  • Figure 4-2 Example of reverse line forming with delay time line for reciprocal methodThe pink line in Figure 4-20 is showing the reduced travel time line generated in Seisimager.It calculates delay time. And optionally, the reverse delay time line is created and does sameprocess and averages the delay time values. With calculated V1 and V2 (when assigned), Itcalculates depth in the each points (P1, P2, … Pn) within reciprocal window according toEquation (2.44) and interpolates those.The result will be shown in Chapter 5. 4.4.2.12 Time term inversionTime-term inversion assumes that the subsurface is vertically stratified and does notconsider the lateral changes during inversion. The depth to the top of the underlying layers iscalculated based on points of first break picking. On the basis of the points assigned fordifferent layers, a layered model is generated. The depth is calculated and interpolated andthe layered model from the time term inversion is generated (Takaya Iwasaki 2002;Seisimager/2D Manual 2005)..In this project, with the values V1 and V2 calculated in Seisimager, depths of every point (P1,P2,.., Pn) in Figure 4-20 are calculated by principle of Equation (2.54) and interpolated.Same process is performed between 2nd layer and 3rd layer if there is 3rd layer.Figure 4-21 shows one example of result of time-term inversion.Junghee Kim 55
  • Figure 4-3 Example of Layered model from time-term inversion (from one move-up data of North Line) 4.4.2.13 Tomographic inversionThe tomographic inversion as discussed in chapter three, tries to match the acquired data byiteratively adjusting a model until the misfit between the data created from this model and thereal data is below some acceptable level. The tomographic inversion performed in thisproject uses an initial model generated from time term inversion (Jacob R. Sheehan et al.2000 ; Seisimager/2D Manual 2005)..Tomographic inversion method is fairly sensitive to the initial model. It was thereforenecessary that out results of time term inversion was good enough to start the tomographicinversion. (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005).Junghee Kim 56
  • Figure 4-4 Process of Tomographic inversion (from one move-up data of North Line)Actual values of matrix To are calculated with layers designed for tomographic inversion.The values of layer lengths get divided and become corresponding to the number of layersdesigned manually to make initial model. To make it clear, let’s assume the number of layers in time-term inversion was 3 and 6layers are designed for tomographic inversion.Junghee Kim 57
  • Figure 4-23 Design of the number of layers for initial model As seen Figure 4-23, number of elements in matrix of To became same number as T1 (From3 layers to 6 layers) and it is applied to find ∆S. The number of elements in matrix of ∆S, S1,S2, ….,Sn becomes same number as the number of layers manually designed fortomographic inversion.In this project, to find sensitivity of initial model by parameter (the number of layers, minimumvelocity and maximum velocity) set up was tested before tomographic inversion.And at the point when ∆Y is almost “0” when RMS values do not decrease much anymore,the number of iterations was checked. (note that RMS values are inversely proportional tothe number of iterations ) The chosen value of number of iterations is n for the tomographicinversion.Setting range of Minimum and maximum velocities were tested.After tomographic inversion, ray tracing was performed to show the penetration of the raysused in estimating the synthetic travel time data employed in the tomographic inversionalgorithm.Junghee Kim 58
  • Figure 4-24 Ray tracing path in tomographic inversionThrough ray tracing path, the reliability of the data with depth was checked. (note that it isnot possible to sample beyond depth not reachable with ray tracing path with the hammerseismic data. (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005)Junghee Kim 59
  • 4.3.3 Seismic Reflection Data Processing and Analysis in North Line Figure 4-25 work-flow of seismic reflection data processing and analysis in North LineTo generate stack that can be compared with image from refraction processing, basicseismic reflection data processing has been performed in Promax.SEG-Y file is used for this process. With the hammer seismic data in Promax, the seismicreflection data processing is performed. Even though the seismic reaches very shallow, itwould be enough to prove the effect of static correction derived from refraction data in thestack. 4.3.3.1 Refraction MutingThe direct arrival waves and refracted waves dominate data. The amplitudes related to thoseevents are high because they travel closely and are not attenuated. (Jakubowicz 2012)In seismic reflection data processing, refraction and direct arrival are considered as acoherent noise and removed. The refraction muting is applied to these data.Junghee Kim 60
  • Figure 4-26 Refraction muting in Promax. (left: before refraction muting, middle: applying refraction muting, right : after refraction muting )Junghee Kim 61
  • 4.3.3.2 Bandpass FilteringBandpass Filter is applied. Here bandpass filter(s) is a frequency filter(s) to each input traceoperated by the filter algorithm in the frequency domain (Steve H. Danbom, Ph.D., P.G. RiceUniversity ESCI 444). To find out the range of frequency of bandpass, the bandpassparameter tests have been conducted.(note that the attempt to find out the range offrequency of bandpass using the function of FK Spectrum Analysis did not work because inthe analysis window, the signal was highly aliased. This is assumed because the samplingrate is too big. The reason of this assumption is because if KMax of data acquired withhammer are not satisfied with Equation (3.1), the data are aliased. (3.1) Here KMax = Maximum frequency (hz) ∆x = sampling rate (s) (Jakubowicz 2012)The sampling rate was checked in Promax. It was 2.5 ms. The Nyquist Frequency is 1/ 2.5×1000 = 400 hz. The data acquired with hammer must have higher maximum frequency thanthis. Figure 4-27 Aliased reflectors of data in FK spectrum analysisJunghee Kim 62
  • Figure 4-5 Schematic drawing on cut range of Bandpass (frequency: 50 – 100 - 200 - 400 Hz)The parameter test was performed. The ranges of frequencies are illustrated in Appendix 9.The bandpass range of 50-100-200-400 was giving the best result keeping reflector the mostand removing the ground roll the most. So this value was chosen.By applying bandpass with frequency range 50-100-200-400, the ground roll wassuccessfully removed and reflector existing in the data seems to reveal.Junghee Kim 63
  • Figure 4-6 Bandpass filter application ( left: gather before applying bandpass, right: gather after applying bandpass 4.3.3.3 Static CorrectionIn this project, the final datum was set as 2259 m and replacement velocity was set at 1700m/s in this project. The final datum 2259 m was chosen with the height around 10 m higherthan the highest elevation. The replacement velocity 1700 m/s was chosen with the averagevelocity value of weathering layer.Junghee Kim 64
  • 4.3.3.3.1 Elevation Statics Analysis in North line. Figure 4-70 schematic geometry for elevation statics with data from first break picks on first-kick of Promax Elevation static correction is calculated as: (3.2) (Khaled Al Dulaijan 2008)In this project, the base of weathering was calculated in Promax with the first break picks onfirst-kick. And the elevation statics have been calculated based on the value, final datumvalue and replacement velocity.Junghee Kim 65
  • 4.3.3.3.2 Datum static correction from tomographic inversion of Seisimager in Promax: Figure 4-8 schematic geometry for datum statics using data from tomographic inversion of SeisimagertLVL is calculated as: (3.3) (Khaled Al Dulaijan 2008)The elevation static correction is calculated as: (3.4) (Khaled Al Dulaijan 2008)Junghee Kim 66
  • The Datum static correction tDatum = tLVL + tETherefore, (3.5) (Khaled Al Dulaijan 2008)In tomographic inversion’s case, h = h0 + h1 + h2 + h3 + • • • • + hn 1, 2, 3, 4 • • • • • nThe h and a values were at different every each shot because those have different numberof layers. The data calculated from tomographic inversion are in Appendix 10 and 11.With the data from tomographic inversion, the LVL statics (refraction statics), elevationstatics and Datum statics has been calculated.Datum statics correction (Elevation statics + refraction statics) is performed in Promax. Thevalues of the number of layers, thickness and velocity were extracted from results oftomographic inversion in Seisimager. Through the values, the LVL (refraction statics) andelevation statics were calculated and datum statics have been calculated. By inputting andapplying the datum statics values in Promax, the datum statics correction has been done.The result applied with this datum statics correction was compared with the result notapplied with the statics correction and applied with the elevation statics correction by amodel from first break picks in Promax. The results will be shown in Chapter 5.Junghee Kim 67
  • 4.3.3.4 Stacking Figure 4-32 Screen showing difficulties on velocity picking in PromaxRed image (high amplitude) was spreading out in the velocity picking window in Figure 4.32.This made velocity picking very difficult. The reason why the amplitude (red) is spread outseems because offset is very short. The difference of velocities between offsets creates thered image which implies high amplitude. However, due to short offset, the differencebetween offsets is almost none. So, it detects all area within offset as high amplitudes ofvelocities. For this reason, NMO by velocity picking was not chosen. Instead, constantvelocity was assumed and based on this, NMO corrections were applied.To make it clearer, in Figure 4-33, range A is assumed to be distance of North Line (504 m).Within this offset, the hyperbolic line of seismic reflection looks almost straight line withinrange of short offset (504 m) in the beginning of the line. The straight line can be explainedas constant velocity. After tests changing the constant velocities, 2500 m/s of constantvelocity has been chosen for Stacking.Junghee Kim 68
  • Figure 4-33 Schematic drawing showing possibility of use of constant velocity for Normal Move Out in short offsetUsing the constant velocity tested (2500 m/s), normal move out is done and the data arethen stacked.Because the reflectivity range is very shallow in the data, the expected reflector is also veryshallow as well. As seen in Figure 4-34 in the gather, only the travel time 0 to 150 ms isexpected to have reflector.Junghee Kim 69
  • Figure 4-34 Expected reflector through a look into gather in PromaxAfter stacking the gathers, reflectors are shown clearly within 50 ms only in Figure 4-35.Figure 4-35 Reflector shown in Brute stack in PromaxJunghee Kim 70
  • FK-filtering and deconvolution were not working well in the processing. The reasons areassumed as follows. (David Forel et al. 2005)Effective deconvolution operators can be difficult to design because of variable sourcesignatures, short trace lengths, and high attenuation in the shallow subsurface. (David Forelet al., 2005) Deconvolution can be more destructive than constructive in many cases ofshallow seismic. FK filter could not be done on the data because the amount of aliasingobserved in the frequency spectrum of the data made it impractical.Post stack bandpass seems to cut too much seismic image including reflector. So, the post-stack bandpass was not applied at this time. The image was clearer to distinguish whenwithout post-stack bandpass4.3.4 Comparison with the other methods (DC-resistivity) Figure 4-36 work-flow of comparison of North Line with DC-resistivity While electrical resistivity methods are not the key focus of this project, observations fromthe refraction seismic are compared to resistivity data during interpretation. The fundamentaltheory of electrical resistivity is therefore reviewed in this project. 4.3.4.1 DC Resistivity Survey Corresponding internal physical rock properties can characterise materials in the subsurface(Torleif Dahlin, 2001). In imaging the distribution in the subsurface, the differences of theproperties between materials play a very important role (Torleif Dahlin 2001). Resistivity candescribe a material’s resistance to the flow of electricity as one property (Torleif Dahlin2001). By the DC resistivity method, current is injected into the ground along a specifiedarray, while electrical potential measurements along the array are performed to characterisehow the resistivity of the subsurface changes laterally and with depth (Torleif Dahlin 2001).By the resistivity values obtained from the geology of the PAGO2 DC-resistivity line, the datawere inverted into a resistivity model. The model mapped potential structures and fluiddistribution in relation to geothermal effect the area (Imperial College London and ColoradoSchool of Mines Geophysics Field Camp 2012).Junghee Kim 71
  • Figure 4-37 SP and inverted resistivity profiles of PAGO 02 (Imperial College London and Colorado School of MinesGeophysics Field Camp, 2012) In this DC resistivity model, the red colour implies high resistivity and blue colour implies lowresistivity which can possibly include a lot of water because water is very highly conductive. (Imperial College London and Colorado School of Mines Geophysics Field Camp 2012)According to the DC resistivity model, the fault is expected. (Imperial College London andColorado School of Mines Geophysics Field Camp 2012) If this assumption is correct, therewould be some similar indication in result from the refraction data because the area wherefault is expected is sharing the area of North Line.As seen in Figure 4-38, the refraction seismic line in North line is not exactly matched withDC resistivity line but they were crossing. By comparing these two, in this project, theattempt to see the similarity and difference between these two was conducted. Figure 4-9 North line area where North line hammer seismic survey line crossing with PAGO 02 DC resistivity survey line (Map is copyright Google Earth)Junghee Kim 72
  • 4.3.5 Advanced refraction analysis (Zen Garden ) Figure 4-10 Work-flow of advanced refraction analysis in Zen GardenIn Zen Garden, the processing is same as in North Line. The description of same processeswas omitted in this thesis and some additional processes are described in this thesis. 4.3.5.1 First break picks of P-wave in Zen Garden At this time, the first break picks were only picked at first kick points (Note that the hammerseismic source is impulsive energy which is minimum phase. So, first break picks would bethe first energy that is detected.) The first break picks have been picked every 3 shot.Junghee Kim 73
  • Figure 4-11 Example of P-wave first break picking on first-kick in Zen Garden 4.3.5.2 S-wave first break picking At this time also, the first break picks were picked at first kick points (Note that the hammerseismic source is impulsive energy which is minimum phase. So, first break picks would bethe first energy that is detected.) In this case, the horizontal geophone received mostly S-waves and the source was applied to semi-horizontal plate which generated mostly S-wave.The first break picks have been picked every 3 shot. So in Figure 4-43, the first break picks of S-wave were same way as previous. However,because the S-wave data were a lot noisier than P-wave data, more QCs were performed.There were some traces which did not include any information. (note trace offset 33) inFigure 4.41. These kinds of traces were killed for first break picks as seen Figure 4-42 andFigure 4-43 show the process of killing a bad trace and picking first break.Note that the location of acquisition of P-wave and S-wave is same.Junghee Kim 74
  • Figure 4-41 Example of the raw data of S-wave in Zen Garden Figure 4-42 Example of choosing bad trace of S-wave in Zen GardenJunghee Kim 75
  • Figure 4-43 Example of S-wave first break picking on first-kick in Zen GardenZen Garden survey line is only 74 m (only one channel) which does not require connectingthe patterns to show whole seismic refraction map.4.3.5.3 Time-term inversion and Tomographic inversion in Zen GardenIn Zen Garden, the time-term inversion and tomographic inversion with P-wave data and S-wave data were performed. The processes are same as those in North Line.The P-wave result was compared with basic analysis result available from Imperial CollegeLondon and Colorado School of Mines Geophysics Field Camp 2012.When elevations are 2141 m, 2131 m and 2121 m in Zen Garden, the P-wave and S-wavevelocities from tomographic inversion have been checked and recorded. Using the values,Vp/Vs values and Poisson’s ratios were calculated. Using the values, lithology includingporosity with different depth (0 m, 10 m, 20 m deep) were anticipated.Junghee Kim 76
  • 4.3.6 Comparison with Ground Penetration Radar (GPR) Figure 4-44 Work-flow of comparison of Zen Garden with GPRThe data acquired in these areas have been compared with the data processed with seismicrefraction data in these areas. 4.3.6.1 GPR (Ground Penetration Radar)GPR (Ground Penetration Radar) - Non-invasive geophysical method. This is using thepropagation of electromagnetic (EM) waves and makes the image of the subsurface. In Barn3 area, the GPR was pulled along the ground. A transmitting antenna emitted a short, highfrequency EM pulse into the ground every 0.05 m. The EM wave was diffracted, reflectedand refracted when a contrast in the dielectric permittivity within the subsurface exists.Reflected waves at the ground surface were continuously recorded by the receiver in Figure4-45. (Marcin Słowik 2012)Junghee Kim 77
  • Figure 4-45 GPR acquisition comprising of the radar components and the analogue interpretation of a radar timesection. Tx: Transmitter, Rx: Receiver (Redrawn from Imperial College London and Colorado School of MinesGeophysics Field Camp 2012)The result of GPR acquired in Barn 3 have been compared with the results of porositydifference from Vp/Vs and Vp in Zen Garden to find out the reason why the porosity isdifferent with different depth. The Barn 3 area is on similar geology with the Zen Gardenaccording to geological map. So, it can give a good comparison with the Zen Garden area.(Imperial College London and Colorado School of Mines Geophysics Field Camp 2012)In Figure 4-46, the red circle is showing Barn 3 area with comparison of Zen Garden (Bluecircle). Red line is showing data acquisition line in Barn 3.Junghee Kim 78
  • Figure 4-46 Barn 3 survey line (red line: SW- NE) cited from Google MapAs seen in Figure 4-47, the Barn 3 area has very thin Mancos Shale layer (some area in thathas no Mancos Shale layer). And so does Zen Garden area. And Dakota Sandstone layerunderlies the Mancos Shale layer. (Imperial College London and Colorado School of MinesGeophysics Field Camp 2012)Junghee Kim 79
  • Figure 4-47 General cross-section of Pagosa Springs showing the location of Barn 3 and Zen Garden. Vertical scale has been exaggerated to show features in larger detail. (ImperialCollege London and Colorado School of Mines Geophysics Field Camp 2012)The schematic map in Figure 4-47 shows the location of Barn 3 (red arrow) and that of Zen Garden (blue arrow).Junghee Kim 80
  • Figure 4-12 General cross-section of the location of GPR acquisition in data acquisition line of Barn 3. Vertical scale has been exaggerated to show features in larger detail. (ImperialCollege London and Colorado School of Mines Geophysics Field Camp 2012)As seen in Figure 4-48, some areas of data acquisition line of the Barn 3 has no Mancos Shale covered and Dakota Sandstone is revealed onsurface. To find out the property (especially porosity) of the Dakota Sandstone, the revealed point is used as GPR analysis. The red circle inthe Figure 4-48 is showing the GPR acquisition point.Junghee Kim 81
  • CHAPTER FIVE. 5.0 RESULTS AND DISCUSSION5.1 Basic refraction analysis in North Line5.1.1Results from Gradient-Intercept method on the North lineFigures 5-1 to 5-6 show the gradient intercept results from first kick, trough and peak. Fromthese results the sensitivity of the travel time sensitivities are seen to be negligible, i.e. anyform of picking used would still give consistent results.Results of Depth models are as follows.Figure 5-1 Depth model generated from picking first break on Peak in PromaxJunghee Kim 82
  • Figure 5-2 Depth model generated from picking first break on First Kick in PromaxFigure 5-3 Depth model generated from picking first break on Trough in PromaxJunghee Kim 83
  • The results of Velocity models are as follows.Figure 5-4 Velocity model generated from picking first break on Peak in PromaxFigure 5-5 Velocity model generated from picking first break on First Kick in PromaxJunghee Kim 84
  • Figure 5-6 Velocity model generated from picking first break on Trough in Promax Average depth values of picks made on peak in general were the biggest and averagedepth values of picks made on first kick in general were the smallest as seen Table 5-1. Similarly, the values of velocities were a bit different. The average velocity values of firstbreak picks made on peak were generally biggest and those made on first kick weresmallest as seen in Table 5-2.Table 5-1 Depth model from basic refraction analysisTable 5-2 Velocity model from basic refraction analysisJunghee Kim 85
  • The shapes of the models (depth model and velocity model respectively) generated fromdifferent first break picking were similar (Figures 5-1 ~ 5-3 and Figure 5-4 ~ 5-6). The factthat similar models are created explains that the position where the first breaks are pickeddoes not affect the results that much. The most important thing on first break picking isconsistency of the first break picks in both cases of depth model and velocity modelaccording to this test. Error analysis (difference between value applied with polyfit) and has been performed(Appendix 7). The results were 3.2 m/s average. This is inaccuracy rate of about 8%. Thisseems because the polyfit function is very sensitive to the range of polyfit that is mademanually. More accurate analyses are required. However, at least, this method gave some information. The findings through results fromdepth models and velocity models were this area (North Line) is undulated and made ofthree layers.5.2 Advanced seismic refraction analysis in North Line As seen in Figure 5-7, this area is shown as 3 layers. In every 3 shot, the travel time hasbeen assigned with 2nd layer and 3rd layer. The first layers were assigned with red colour,the 2nd layers were assigned with green colour and 3rd layers were assigned with bluecolour.5.2.1. Time Term Inversion The time-term inversion method generated a model with about 30 m depth to the bedrockrefractor. The RMS error were less than 2 ms. To make comparison with reciprocal methodeasier, (vertical) smoothing effect of the layers was added to the time-term inversion. Notethat this is still layered model having constant velocities in each layer. By giving smoothingeffect, possible change of velocities with tomographic inversion (but just image function ofprediction) is shown on the time-term inversion. To do this, parameter of number ofsmoothing passes has been set up as 3. Here, the bigger is number, the smoother is image.The image of the result of the time-term inversion is shown in Figure 5-8.Junghee Kim 86
  • Figure 5-7 Connected Layer assignment of whole North line in Plotrefa TM of SeisimageJunghee Kim 87
  • Figure 5-8 Layered model from time-term inversion of North line with smoothing effect (Smoothing passes: 3) added in Plotrefa TM of SeisimagerJunghee Kim 88
  • 5.2.2 Tomographic Inversion The tomographic method needs the input of an initial velocity model. In this project, theinitial model was used with the velocity model from time term inversion. Here the reason designing the number of layers for the initial model is because Length ofray path gets different depends on this value as seen Figure. Figure 5-9 Principle of designing the number of layers for the initial modelThe difference between S0 values (Slowness: 1/V1, 1/V2, 1/V3) from time-term inversion andnew S1 values (calculated) is calculated. At this time, the number of elements of matrix S1becomes same number of layers designed for tomographic inversion. Please refer to Figure5-9. (4.1)(Here, L = matrix of lij and T1 = calculated travel time, To= observed travel time) (4.2)Because it is non-linear problem, Lt L × ∆S = Lt × ∆T So, ∆S is found. And initial model isdecided as So + ∆STherefore,Junghee Kim 89
  • (4.3)One part of move-ups in North Line in Figure 5-10 was used to test the parameters forgenerating an initial model.Junghee Kim 90
  • Figure 5-10 the image of one move-up time term inversion result chosen for parameter tests for initial model in North line in comparison with the whole North line time terminversion image in Plotrefa TM of SeisimagerJunghee Kim 91
  • Figure 5-11 images of one pattern time term inversion result chosen for parameter tests for initial model in North linein Plotrefa TM of Seisimager ((a) P-wave velocity 30 m/s – 3000 m/s, the number of layers 10 (b) P-wavevelocity 30 m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 3000 m/s, the number oflayers 18)First, as seen in Figure 5-11, with a constant P-wave velocity of minimum 30 m/s andmaximum 3000 m/s, the number of layers was changed. 10, 15, 18 layers were applied togenerate starting models with P-wave velocity (30 m/s ~ 3000 m/s). A bit slightly coarservelocity grids have been shown in the initial model of fewer layers (10) as seen (a) in Figure5-11. But (b) in Figure 5-11 is almost same as (c) in Figure 5-11.Junghee Kim 92
  • Figure 5-12 images of time term inversion result chosen for parameter tests for initial model in North line in PlotrefaTM of Seisimager ((a) P-wave velocity 30 m/s – 1000 m/s, the number of layers 15 (b) P-wave velocity 30m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 10000 m/s, the number of layers 15)For 15 layer starting model fixed, the maximum velocity has been changed to 1000 m/s,3000 m/s and 10000 m/s.1000 m/s maximum velocity setting caused error as seen (a) in Figure 5-12. It seemsbecause the maximum velocity setting value is lower than actual maximum value (around2800 m/s). Note that setting minimum velocities higher than 300 m/s brought error as well. This seemsbecause the minimum velocity setting value is bigger than actual minimum velocity value. (b)and (c) do not have a big difference. Through this test, it is found that setting the maximumvelocity should be higher than actual maximum value and the minimum velocity settingshould be lower than actual minimum value. Therefore, since sufficient flexibility in the number of model layers ( around 15 ) allows thevelocity estimate to be fairly stable, the 15 was chosen for the number of layers, 3000 m/sfor maximum velocity and 30 m/s for minimum velocity were chosen as parameters togenerate the initial model. The Figure 5-13 shows the initial model generated from time-term inversion with parameterof 15 in the number of layers and 3000 m/s in maximum velocity chosen as parameters setJunghee Kim 93
  • in this project.Junghee Kim 94
  • Figure 5-13 The image of initial model in whole North line in Plotrefa TM of Seisimager ( calculated with parameters of P-wave velocity 30 m/s – 3000 m/s, the number of layers 15Junghee Kim 95
  • Figure 5-14 Misfit between synthetic and observed travel time as a function of the iteration number. Observe the lackof significant reduction in the travel time misfit after about 10 iterations. Several tests were performed to decide the number of iterations used in the tomographicinversion. Initial tests show little of no improvement in the velocity model after 10 iterations(Figure 5-14), suggesting possible convergence of the inversion to the true model. Thenumber of iteration was therefore fixed at 10 for application to the entire North line. In this project, number 10 is chosen for the number of iteration in tomographic inversionbecause this number is enough to make misfit between calculated values and observedvalues almost minimum.This image from tomographic inversion in Figure 5-15 is showing more specific and precisegeological feature than time-term inversion providing smooth velocity changes with depth. From this ray tracing path in Figure 5-16, the data up to about 20 m deep would be reliablesince the ray path is going through by the depth. However, after the depth, the ray path doesnot reach which means the data deeper than about 20 m cannot be reliable. Tomographic inversion allowed to confirm the results of the time term inversion and madethe velocities of the sediments and bedrock constrained. But it is allowed to show someinteresting look at possible drainages and shallow low velocity layers. The P-wave velocityof the middle layer in North line area is matched with water saturated sandstone. However,to make sure it is really water saturated sandstone, further investigation is required.Junghee Kim 96
  • Figure 5-15 the image of P-wave velocity model from tomographic inversion in whole North line in Plotrefa TM of Seisimager (value 10 was chosen for the number of iteration )Junghee Kim 97
  • Figure 5-1 the image of Ray tracing path of P-wave velocity model from tomographic inversion in whole North line in Plotrefa TM of SeisimagerJunghee Kim 98
  • Table 5-3 Seismic Velocities of Earth Materials (Gary Mavko 2005)5.2.3 Reciprocal MethodThe reciprocal method generated better image than the time-term inversion data yielding adepth closer to 25 m. Not all the shots were useful in producing the model because thismethod can be applied in the condition that the reciprocal time is same as original timewhich is not easy to be in real data. For this method, manual adjustment of the travel timewas inevitable. And this method was able to calculate delay time only within reciprocal timewindow area and generated the image of velocity model only this area. It is very tedious andredundant job to apply this method in whole survey area while this only shows similar resultto time term inversion data. In other words, application of the reciprocal method is not easyin terms of the process and time required. The reciprocal method slightly improved than themodel by time term inversion. Notice that the reciprocal method generated two layeredmodel (even if it has smooth effect shown in the image) while time-term inversion generatedthree-layered model. This is because the delay time for calculation of depth is onlycalculated with V1 and V2 according to Equation (2.44). In Figure 4-20, The V1 = 300 m/s, V2= 2833 m/s are calculated.After checking this method generates similar result to time-term inversion data and it is notproper for situation of more than three layers, this method is not chosen for furtherinvestigation.Junghee Kim 99
  • Figure 5-2 the image of reciprocal method showing delay time line and reverse time line in one move-up of North line in Plotrefa TM of Seisimager (delay times in both sides are calculated and averaged )Junghee Kim 100
  • Figure 5-3 the image of P-wave velocity model generated by reciprocal method in one move-up of North line inPlotrefa TM of Seisimager ( delay times in both sides are calculated and averaged )The delay times calculated in both sides and averaged. The values averaged with bothdirections helps buffering the error that can be caused in one side with the other side’s resultin some cases.Note that even if the result of reciprocal method looks several colour gradients, this isbasically two layered model. The gradients were generated by Smoothing function ofSeisimager to predict real velocity model.Junghee Kim 101
  • Figure 5-4 Comparison between images of P-wave velocity models generated by reciprocal method and time-terminversion in one move-up of North line in Plotrefa TM of Seisimager (Note that both methods are conducted in sameposition)Junghee Kim 102
  • 5.3 Statics analysis of P-wave data in North Line5.5.1 Elevation static correction from first break picks picked in Promax:The graphs in Figure 5-20 and Figure 5-21 show the elevation statics calculated from firstbreak picks on first kick picked in Promax.Figure 5-20 plots of Elevation static correction on P-wave obtained from first break pick on first kick in North Lineof receiver shown in Promax.Figure 5-11 plots of Elevation static correction on P-wave obtained from first break pick on first kick in North Lineof source shown in Promax .Because the location of source and receiver in North Line is shared, the shapes of elevationstatics are identical.Junghee Kim 103
  • 5.5.2 Datum statics from tomographic inversion .On the other hand, datum statics has been calculated with data from tomographic inversionderived in Seisimager. The data are available in Appendix 10 and 11. To calculate theDatum statics, the LVL statics (refraction statics) and elevation statics are also calculatedfrom the data from tomographic inversion. The results are shown in Figure 5-22.Figure 5-22 Values of LVL Static ( refraction static), Elevation static correction of receiver and total datum staticcorrection shown in Promax. The values of elevation static correction and LVL static correction are added up to finddatum static correction.The datum statics inputted in Promax are shown as shown in Figure 5-23 and Figure 5-24.The Figure 5-23 is showing receiver datum static correction.Junghee Kim 104
  • Figure 5-23 plots of Datum static correction on P-wave obtained from tomographic inversion in North Line ofreceiver shown in Promax.Source datum static correction is shown in Figure 5-24.Figure 5-24 plots of Datum static correction on P-wave obtained from tomographic inversion in North Line of sourceshown in Promax.Because the location of source and receiver in North Line is shared, the shape of datumstatics is identical.Junghee Kim 105
  • 5.5.3 Application of static correction to the stackAfter applying statics, there was slight change. Since the reflector is not deep enough, it wasnot easy to distinguish the change. However there were very small changes after applyingstatic corrections. Static corrections helped the reflectors not cut by bringing the reflectordown from upper top window. And datum static correction gave the best image helpingcontinuity of the seismic.Stack not applied with any static correction (but bandpass applied) in North line area isshown in Figure 5-25.Figure 5-25 the image of stack not applied with static correction (only bandpass applied : Bandpass frequency range :50 – 100 -200 -400 hz).With elevation statics and the datum statics respectively were applied in gathers of seismicdata of North line in Promax and stacked and compared with brute stack which is not appliedto any statics.Stack applied with Elevation static correction calculated with first break picked in Promax isshown in Figure 5-26.Junghee Kim 106
  • Figure 5-26 the image of stack applied with elevation static correction ( bandpass and elevation static correctionapplied : Bandpass frequency range : 50 – 100 -200 -400 hz).Stack applied with datum static correction (Datum statics (refraction (LVL) statics + elevationstatics) and Bandpass) is shown in Figure 5-27.Figure 5-27 the image of stack applied with Datum static correction ( bandpass and datum static correction appliedapplied : Bandpass frequency range : 50 – 100 -200 -400 hz Here Datum static correction = LVL static correction (Refraction static correction(LVL))The Images of Figure 5-26 and Figure 5-27 compared to Figure 5-25 were improved in termsof continuity of the data. The datum statics (Figure 5-27) seems to show the best image.Elevation statics (Figure 5-26) seems to be showing second best image.The following Figure 5-28, Figure 5-29 and Figure 5-30 are showing magnified stacks whichare applied with elevation static correction and datum static correction. And the first stack isnot applied with any static correction.Junghee Kim 107
  • Figure 5-28 the image of stack ( only bandpass applied : Bandpass frequency range : 50 – 100 -200 -400 hz .Figure 5-29 the image of stack applied with elevation static correction (bandpass and elevation static correction applied : Bandpass frequency range : 50 – 100 -200 -400 hz .)Figure 5-30 the image of stack applied with datum static correction (bandpass and datum static correction applied: Bandpass frequency range : 50 – 100 -200 -400 hz Here datumstatic correction = LVL static correction ( Refraction static correction ) + elevation static correction.Junghee Kim 108
  • The reason why these are magnified is to see the difference closer because the change bystatic correction was very small. As seen in Figure 5-28, the stack that is not applied withstatic correction was the roughest. However elevation static correction and datum staticcorrection made the stack in the window adjusting the continuity in window. The elevationstatic correction drew the image down the most and the datum static correction drew theimage down but less than elevation statics. In terms of continuity, the datum staticcorrection seems to make the stack the best image even if the change is very slight. Thisseems that refraction static correction played a role of correction of brute stack.For datum statics, the velocity model created from tomographic inversion was used. It gavean improvement in the image. But still it is not clear even if it gives some indication ongeology. The biggest reason of this seems because the reflector depth is not that deepenough to show geological feature.5.5.4 Comparison of the stack with results from refraction analysis.As seen in Figure 5-31, the time term inversion and tomographic inversion result and thestack result from reflection processing indicate a possible fault line. However, the F1 is justguess based on the results. More certain indication is required to certify this. Later, DC-resistivity survey result will be shown to support this finding.Figure 5-32 is showing the stack applied with datum static correction and depth converted.The stack is superimposed with refraction line (image of tomographic inversion). In this case,the refraction line is assumed to be flat to compare with brute stack.Through this comparison, the depth of the weathering layer of the tomographic inversion canbe compared with reflection image. The weathering layer is just up to around 15 m deep asseen in Figure 5-32. The possible fault (F1) can be indicated. However because the stack isshowing brief geological feature, it needs more investigation.Junghee Kim 109
  • Figure 5-31 A possible fault by comparison between refraction processed image and reflection processed image inNorth Line. (a) the image from time-term inversion (b) the image from tomographic inversion (c) the image from thestack applied with datum statics correction.Junghee Kim 110
  • Figure 5-32 A possible fault (F1) by comparison of the stack with superimposed and flattened refraction processed image ( from tomographic inversion) in NorthLine Junghee Kim 111
  • 5.5.5 Comparison with the result of DC-resistivity survey in North line area.One DC-resistivity survey line crossing North line of seismic refraction line was considered tobe compared with North line to make sure on the fault. Because in distance 1050 m W-E ofDC-resistivity survey line, a possible fault was interpreted, in North line, around the area,there should be some indication of fault in North line hammer seismic survey line as wellbecause DC-resistivity survey line and hammer seismic survey line is closely located andcrossing in some point..As seen in Figure 5-33, similarity of the location of a possible fault was detected in betweenDC survey line and seismic refraction survey line. The location of fault is matched. The faultis very likely to be there based on results of the refraction analysis, stack analysis and DC-resistivity model analysis...Junghee Kim 112
  • Figure 5-33 A possible fault expected by result from DC-resistivity survey and Hammer seismic survey in North Line area (The DC-resistivity model is fit to the PAGO02 pararelly,and the tomographic inversion image is fit to the North line in parallel) DC-resistivity image is cited from Imperial College London and Colorado School of Mines Geophysics Camp2012.Junghee Kim 113
  • 5.4 Advanced refraction analysis in Zen GardenFor this, Zen Garden near North line has been investigated with S-wave and P-waveacquisition data. It could help to understand the possible presence of groundwater and moreaccurate information through Poisson’s ratio and Vp/Vs in this area.5.4.1 P-wave velocity model analysis in Zen GardenP-wave velocity model result of Time-term inversion in Zen Garden is shown in Figure 5-34.RMS error was 2.20 msec (These data are reliable since the RMS error are less than 5msec). Just like North line, the layers were shown as three layers and middle layer seemssimilar to velocity of water-saturated sandstone (about 1500 m/s) please refer to the chart..Just like North line, tomographic inversion has performed using the result from time terminversion for an initial model. Before performing the tomographic inversion, RMS valuechanges by change of the number of iteration have been checked. After 10 iterations, theRMS value has been stabilised.Figure 5-34 the image of P-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TM ofSeisimagerFor this reason, value of 10 is chosen as a parameter for the number of iteration intomographic inversion. This number is enough to make misfit almost minimumThe image of the result from tomographic inversion is shown in Figure 5-35. The result fromtomographic inversion shows more smoothed velocities while results from time-terminversion show only constant velocities in each layer.Junghee Kim 114
  • Figure 5-35 the image of P-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration)At this time RMS error was 2.3 ms. In P-wave analysis, there seem to have water saturatedsandstone showing layer around 1500 m/s according to Table 5-3.Bottom layer in these images seems like sandstone because the velocity is matched withvelocity of the sandstone around 2400 m/s in Table 5-3. However, the data are not reliable tomake sure this is sandstone because this depth is not reachable for ray tracing path as seenin Figure 5-36.Area deeper than 15 m should needs further investigation for this reason. To investigate thedeeper area, the other method such as DC-resistivity survey would be helpful. In fact,processing of deep seismic data at a nearby site suggests that the velocity of the underlyingDakota Sandstone is 2400 m/s (Karalis 2012)Junghee Kim 115
  • Figure 5-16 the image of Ray tracing path of P-wave velocity model from tomographic inversion in Zen Garden inPlotrefa TM of SeisimagerRMS = 3.82 ms (the value is less than 5, these data are reliable enough )In brief, P-wave analysis showed there would be water saturated sandstones range betweendepth 2 m and depth 13 m according to Table 5-3.Junghee Kim 116
  • Figure 5-37 Comparison of P-wave velocity model from tomographic inversion and subsurface model from basic gradient intercept method done by Imperial College London andColorado School of Mines Geophysics Field Camp, 2012 (right Figure - ( Imperial College London and Colorado School of Mines Geophysics 2012 )When comparing the P-wave velocity model result of tomographic inversion with the basic hammer seismic analysis which is done in Coloradofield camp 2012 in the Zen Garden, there were some similarities on approximate. One of differences was in the basic hammer seismic analysis,the water saturated sandstone. (Depth 2 m – 13 m) is interpreted as saturated sediment. According to the results from S-wave, Vp/Vs analysisand Poisson’s ratio analysis (this will be shown soon), the middle part (depth 2 m - 13 m) seems to be water saturated sandstone.Junghee Kim 117
  • In Zen Garden area, S-wave data have been acquired. The S-wave velocity model showedthe S-wave was slower than P-wave. However, the shape of the S-wave velocity model wasvery similar to P-wave velocity model. The information of S-wave with that of P-wave insame location was very useful to find out information of lithology and porosity.Just like P-wave, Time term inversion has been performed with S-wave. The result is shownin Figure 5-38.Figure 5-38 the image of S-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TM ofSeisimagerThe image of result from tomographic inversion of S-wave data using the initial modelderived from the time-term inversion is shown in Figure 5-39.S-wave did not detect the top layer in the time inversion. However, the velocity 780 m/s of S-wave in the middle layer is matched with water saturated sandstone and the velocity 1100m/s of S-wave in bottom layer is matched with sandstone according to the Table 5-3. .Figure 5-39 the image of S-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration)Junghee Kim 118
  • 5.4.2 S-wave Velocity model from tomographic inversion in Zen GardenAfter tomographic inversion, the top layer seems to appear. The lower velocity (about 400m/s) in depth 0 m to 2 m suggests shale and the higher velocity (about 700 m/s ) in depth 2m to 12 m suggests sandstone according to Table 5.3, which is consistent with thegeological model of the area.Figure 5-40 the image of Ray tracing path of S-wave velocity model from tomographic inversion in Zen Garden inPlotrefa TM of SeisimagerSimilarly, the range of depth between 0 m and 10 m in S-wave result can be reliable.However, when deeper than that range and the ray tracing path did not reach, the data is notreliable. The further investigation is required in the deeper area.Junghee Kim 119
  • Figure 5-41 Comparison of shapes of P-wave data and S-wave dataAs seen in Figure 5-41, the shapes between image from P-wave and image from S-wave arevery similar. Just the velocities are different. This can be interpreted as the water existing inZen Garden is saturated in the pores of rocks through which S-wave can still go througheven if the S-wave is blocked by water. Note S-wave cannot go through water or air.Junghee Kim 120
  • 5.4.3 Poison’s ratio analysisTable 5-4 P- to S-wave velocity and Poisson’s ratios calculated from P- and S-wave in Zen GardenThe Table 5-4 is calculated by the Equation (4.4). The Poisson’s ratio can be expressed interms of properties measured in the field including P-wave velocity and S-wave velocity(Thomas Brocher 2005) Here, Ơ = Poisson’s ratio Vp = P-wave velocity Vs = S-wave velocityNote that if Vs = 0, Poisson’s ratio becomes 1/2. This is indicating either a fluid becauseshear waves do not pass through the fluids or a material that maintains constant volumeregardless of stress. This is known as an ideal incompressible material. Vs approaching 0 ischaracteristic of a gas reservoir. The Poisson’s ratio for Carbonate rocks is ~ 0.3, forsandstones is ~ 0.2, and above 0.3 for shale. The Poisson’s ratio of coal is ~ 0.4. (cited fromG. N. Greaves et al. Poisson’s ratio and modern materials; Nature material.)In this case of Zen Garden as you can see in Table 4.2, in the top area of elevation 2141, Ơ= 0.43 which is bigger than 0.3. Therefore, it indicates this part can be made of shale. This ismatched with information of geology in this area. The top part is covered with Mancos Shaleaccording to geophysics summer camp report (Imperial College London and ColoradoSchool of Mines geophysics field camp 2012). However, there are still possibilities the top ofZen Garden can be made of different materials. Further investigation on this part is requiredto make sure this is Mancos Shale layer.Junghee Kim 121
  • Figure 5-42 Chart of Poisson’s ratio, Vp/Vs ratio and P-wave velocity (Redrawn from Thomas Brocher 2005)This chart is showing the lithology based on the Poisson’s ratio, P-wave velocity and Vp/Vsratio. As seen figure 4-32, the red dot is when elevation is 2131 m, Vp = 1500 m/s Vs = 900m/s and Vp/Vs is around 1.67. As seen in Fig 1, the location of red dot is very close to theSandstond (fluid). This means this might be water saturated sandstone which is matchedwith result of P-wave analysis in the elevation area (1500 m/s P-wave velocity).The black dot is when elevation is Vp = 1800 m/s, V= 1222 m/s, Vp/Vs = 1.47 and Poisson’sratio = 0.073. This is very closely located near Sandstone (gas). That is matched withgeology reported by Imperial College London and Colorado school of mines in the summerfield trip as Dakota Sandstone.Junghee Kim 122
  • 5.4.4 Vp/Vs analysis Figure 5-43 Chart of Vp, Vp/Vs ratio and Porosity in Zen Garden ( Redrawn from Ross Crain 2000)As seen in Figure 5-43, the red dot is showing when elevation is 2131 m, Vp = 1500 m/s Vs= 900 m/s. The value of the Vp/Vs is around 1.67. The porosity in this case is very high.This can imply that the water is saturated in the big porosities. In the other hand, the blackdot is showing when elevation is 2121 m, Vp = 1800 m/s, V= 1222 m/s, Vp/Vs = 1.47 andPoisson’s ratio = 0.073. This case, the porosity is lower than the case of red dot. That meansthis does not include water as much as the case of red dot. So, this can indicate that thedeeper the less porosity ( where water can be saturated ) exist.GPR result in Barn 3 area which is assumed to be similar geological structure with ZenGarden from Colorado field camp has been analysed. The result of GPR is shown in Figure4-81.Junghee Kim 123
  • Figure 5-2 (a) Seismic section at Barn 3 and (b) its interpretation related to the Dakota Sandstone. (Imperial CollegeLondon and Colorado School of Mines Geophysics 2012)In the Figure 5-44, the irregularity in the reflectors is shown because of the presence of voidspaces (porosity) that allow radar waves to pass through. In depth 1 m, fractured Dakotasandstone has been detected by the GPR. As seen in Figure 5-35. the irregularity of thereflectors is suddenly getting a lot after 1 m, and gets less and less. That means porosity isgetting smaller and smaller as it gets deep.Interpretation on this is as follows. As seen in Figure 4-34, in image from GPR weatheredsandstone was detected on top of unweathered sandstone. It tells the weathered sandstonewould have more porosity and possibly water saturated. And as the depth is more deeper,more unweathered sandstones are gradually more dominating the part. So, it would be lessporosity and less water saturated. This can explain about reason of porosity result fromVp/Vs and Poisson’s ratioJunghee Kim 124
  • CHAPTER SIX. 6.0 Conclusions and RecommendationsGradient-intercept, reciprocal, time-term inversion and tomographic inversion have beenused to interpret first arrival travel time picks of refractions data from the Pagosa Springs,Colorado, USA. The various results show a strong correlation to the advanced seismicrefraction data even if the time-intercept method is just an approximation. The time terminversion gave more specific and precise information on the area. And the tomographicinversion used the result from time term inversion and reduced the misfit between observeddata and calculated data and gave more realistic information on the area.Reciprocal method uses the so-called delay time method and was used on certain part of theNorth Line. Results show that it was sensitive to the presence of significance refractor over-lapping the reciprocal time window. Moreover, it was computationally expensive as repeatedexperiments needed to be done. However, the result was a bit better than time terminversion as it should a gradation in the velocity values with depth. The reciprocal methodproduced only 2 layered models while time-term inversion produced 3 layered models eventhough it has showed smoothed effect in one of the function of Seisimager with which realvelocity model can be predicted.In general, the results of modelling in both Zen Garden area and North Line area seem to beagreeing with existing information from Colorado Geophysics Camp. However, the resultsfrom tomographic inversion gave better insight on North Line and Zen Garden. Since inmany cases, the real earth does not change the layers directly from one constant velocity todifferent constant velocity directly, the tomographic inversion image gave more realistic andgradually changing velocity model. According to the P-wave & S-wave analysis in ZenGarden, and P-wave velocity analysis in North Line and Zen Garden, a layer of watersaturated sandstone was interpreted. According to Poisson’s ratio, Vp/Vs and P-wavevalues, the lithology of top layer is possibly shale. This is matched with Mancos Shale ingeology information provided in geological map. The middle part is matched with watersaturated sandstone which was same result from previously processed seismic refractiondata. This part was all same results in North Line and Zen Garden. (Appendix 1, 2, 3, 4)As for porosity, based on Poisson’s ratio and P wave velocity values, after passingunconsolidated sediment (which is very thin), the porosity was high but shows a decreasewith depth. This is corresponding to result of GPR that was done previously in ImperialCollege London and Colorado School of Mines geophysics field camp 2012. This porositytheory supports the water saturated zone in shallow area of Sandstone. (Dakota sandstoneaccording to geological information provided in geological map)In general, the P-wave velocities found in Pagosa Springs area range from 400 m/s toapproximately 2500 m/s with a calculated depth of average 15 m. This was similar values inboth Zen Garden and North Line. These areas share the same geology structure becausethey are geographically very close according to the provided geological map from ImperialCollege London and Colorado School of Mines Geophysics Field Camp 2012. This explainswhy the P-wave velocity models are so similar.Bedrock velocities of P-wave in this area range from 2000 m/s to 2700 m/s in both the Northline and Zen Garden. However, the information of Bedrock is found difficult to trust becausethe ray tracing path did not reach the depth. For investigation of the depth of the depthJunghee Kim 125
  • further investigation is required, perhaps this time with a longer survey spread and astronger energy source.Both model of North Line and model of Zen Garden suggest a maximum possible shale layerthickness of 1 m to 2 m. Irregularities and dipping interface in the bedrock surface wereslightly shown in the both models.The north Line shows a consistent sediment thickness with a bit of thickening around adistance of 170 m to 220 m implying there might be a possible fault in around distance 220m area matching with location of fault interpreted by DC-resistivity survey. In Zen Garden,the weathering layer was gradually deepening from South to North and the bottom interfacewas showing as dipping and slightly undulated.The datum statics correction and elevation statics correction both gave slight improvement tothe reflection image after brute stack. Even if it is very small difference, the datum staticscorrection using data obtained from tomographic inversion gave a little improvement to thestack than elevation statics obtained from data of first break picks on first-kick. However,because reflector in the stack was too shallow, significant difference by the statics correctionwas not shown much.Through the comparison between results from time-term inversion, tomographic inversionand stack, similar feature of possible fault was found. This fault line is matched with existingresult from previously conducted DC resistivity survey. Therefore, there is a possible fault inNorth Line.If the deep seismic was done in the same line, the datum statics calculated would givesignificant improvement of the deep seismic data. And through the deep seismic survey, theinvestigation under the depth limit of refraction survey would be done. More data such asdensity, log data and etc would give more advanced investigations. The different inversionsuch as wave-form inversion would be worth trying to have better and more precise imagesof the interest area.Junghee Kim 126
  • ReferencesAnne Obermann, 2000. Part I: Seismic Refraction [Online] Technical course report Availableat:http://isterre.fr/spip.php?action=acceder_document&arg=2350&cle=4ba273eac19b9e0ddd3c961ea3b0de7573e083f7&file=pptx%2FSeismic_Refraction_for_class_2.pptx [accessed 11June 2012]Chung-Kuang P. Chu and Jerry M. Mendel, 1994. First Break Refraction Event PickingUsing Fuzzy Logic Systems, Vol. 2 p 1 - 4, IEEE.Cox M. J., 1999. Static Corrections for Seismic Reflection Surveys. Soceity of ExplorationGeophysicists. Vol 1 p 1 - 546Gary Mavko, 2005. Conceptual Overview of Rock and Fluid Factors that Impact SeismicVelocity and Impedance, Parameters that Influence Seismic Velocity, Stanford Rock PhysicsLaboratory. [Online] Technical report Available at:http://webcache.googleusercontent.com/search?q=cache:2aRaqDL5kQsJ:pangea.stanford.edu/courses/gp262/Notes/8.SeismicVelocity.pdf+&cd=1&hl=ko&ct=clnkCraig Lippus, 2007. Fundamentals of Seismic Refraction, Theory, Acquisition, andInterpretation, Short Course Presentation data, Seismic products Geometrics, Inc. San Jose,California, USA [Online] Technical PPT report. Available at:ftp://geom.geometrics.com/pub/seismic/SeisImager/ShortCourse/refraction%20short%20course2.pptDavid Forel et al., 2005. Seismic Data Processing with Seismic Un*x: A 2D Seismic DataProcessing Primer; Chapter 14.f-k Filter and Deconvolution, Society of ExplorationGeophysicists. Vol 1 p 1 - 288.Dobrin M. B., 1976. Introduction to Geophysical Prospecting; 3rd edn: McGraw-Hill Book Co.,Inc, New York. Vol.1 p 1 – 630.Fred C., 1982. A Technical Assistance Panels Program Report: Pagosa Springs LandfillEvaluation, U.S. Environmental Protection Agency. Hart Associates, Inc. Denver, Coloradoavailable at:http://nepis.epa.gov/Exe/ZyNET.exe/91008KSI.txt?ZyActionD=ZyDocument&Client=EPA&Index=1981%20Thru%201985&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C81THRU85%5CTXT%5C00000015%5C91008KSI.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1 [accessed 10. July. 2012]Hagedoorn, J.G., 2006. The plus-minus method of interpreting seismic refraction section,Geophysical Prospecting, Volume 7, Issue 2, Page 158 – 182, June 1959.Imperial College London and Colorado School of Mines, Geophysics field camp, 2012.Geophysical Characterization of the Geothermal System in Pagosa Springs Area, UpperSan Juan Basin, Archuleta County, Colorado, Geophysics Field Camp 2012, Volume 1, P 1Junghee Kim 127
  • – 303.Jacob R. Sheenhan et al., 2000. Application of Seismic Refraction Tomography to KarstCavities, Oak Ridge National Laboratory, Oak Ridge, U.S. Army Environmental Center,Aberdeen, MD 21010. Volume 1. P1 - 391Jacob T. Fokkema and M.Nafi Toksoz, 2012.Journal of Seismic Exploration, SeismicApplications book series. Volume 1, p 1- 508Jakubowicz, H., 2012, Advanced Seismic Method. MSc Petroleum geophysics Lecture atImperial College LondonJocelyn Dufour and Darren S. Foltinek, 2000. Analysis by basic reciprocal methodKaralis, P., 2012. Seismic investigation of Pagosa Springs Colorado, MSc Thesis, ImperialCollege LondonKarastathis V.K. et al.. 2007. Application of shallow seismic techniques in the study of activefaults: The Atalanti normal fault, central Greece. Journal of Applied Geophysics 62 p 215 –233Khaled Al Dulaijan, 2008. Near-surface Characterization Using Seismic Refraction andSurface-wave Methods, University of Calgary. Calgary, Alberta.Marcin Slowik, 2012. Influence of measurement conditions on depth range and resolution ofGPR images: The example of Iowland valley alluvial fill (the Obra River, Poland). Journal ofApplied Geophysics, Volume 85, Pages 1-14,Mathworks, 2012. [Online] Technical book, Available at:http://www.mathworks.co.uk/products/matlab/Phillip Kearey et al., 2002. An introduction to Geophysical Exploration, Blackwell Science.,Volume 1, p1-503.Promax / 2D Seismic Processing and Analysis, 1998. Rev. B, Copyright, Landmark GraphicsCorporation [Online] On-line technical manual, Available athttp://www.google.com/url?sa=t&rct=j&q=promax%20%2F%202d%20seismic%20processing%20and%20analysis%2C&source=web&cd=1&ved=0CCQQFjAA&url=http%3A%2F%2Fimages.otnayirt.multiply.multiplycontent.com%2Fattachment%2F0%2FSWq2kwoKCh8AAAXZp7A1%2Fpromax2dtxt-e.pdf%3Fkey%3Dotnayirt%3Ajournal%3A7%26nmid%3D166958817&ei=BTc8UL3YBOma1AW06YHABw&usg=AFQjCNGI1rOva1gQststm3J2Xpkp1hRTPgRoss E.R. Crain, 2000. Crain’s Petrophysical Handbook, [Online] On-line SharewarePetrophysics Training and Reference Manual, Available at http://www.spec2000.net/01-index.htmSeisImager /2D Manual, 2005. Version 3.1. [Online] On-line technical manual, Available athttp://www.google.com/url?sa=t&rct=j&q=seisimager&source=web&cd=17&ved=0CFIQFjAGOAo&url=ftp%3A%2F%2Fgeom.geometrics.com%2Fpub%2Fseismic%2FSeisImager%2FInstallation_CD%2FSeisImager2D_Manual%2FSeisImager2D_Manual_v3.3.pdf&ei=azc8UNbwF8SX1AWKtoG4Aw&usg=AFQjCNEdeaToM1V9w8x50ubsp9s2e3p7PQSheriff R. E., 2002. Encyclopedic Dictionary of Applied Geophysics. Soceity of ExplorationJunghee Kim 128
  • Geophysicists. Geophysical Vol. 13,Takaya Iwasaki, 2002. Extended time-term method for identifying lateral structural variationsfrom seismic refraction data, Earth Planets Space, 54, 663-677, Earthquake ResearchInstitute, the University of Tokyo, Yayoi 1-1-1, Bunkyo-ku Kokyo , Japan.Thomas M. Brocher, 2005. Empirical Relations between Elastic Wavespeeds and Density inthe Earth’s Crust, Bulletin of the Seismological Society of America. Volume 95, no. 6 p.2081-2092.Torleif Dahlin, 2001. The development of DC resistivity imaging techniques, Volume 27,Issue 9, Pages 1019-1029, Geological Applications of Digital Imaging, Department ofGeotechnology, Lund University, Lund, Sweden.Junghee Kim 129
  • Appendix1. Results of time-term inversion in North Line and Zen Garden2. Results of reciprocal method in North Line and Zen Garden3. Values Vp and Vs and lithology depending on the different depth in NorthLine and Zen GardenJunghee Kim 130
  • 4. Results of tomographic inversion in North Line and Zen GardenJunghee Kim 131
  • 5. Data QC in North Line.Junghee Kim 132
  • 6. Elevation data in North Line.Junghee Kim 133
  • 7. Error analysis between values of actual plots and values derived from polifit(one example of SIN 4 ) - AJunghee Kim 134
  • 7. Error analysis between values of actual plots and valuesderived from polifit (one example of SIN 4 ) - B7. Error analysis between values of actual plots and values derived from polifit(one example of SIN 4 ) - CJunghee Kim 135
  • 8. Reciprocal time check -AJunghee Kim 136
  • 8. Reciprocal time check -BJunghee Kim 137
  • 8. Reciprocal time check -CJunghee Kim 138
  • 9. Parameter test for bandpassJunghee Kim 139
  • 10. Statics data obtained from tomographic inversion in Seisimager (Calculation)Junghee Kim 140
  • 11. Statics data obtained from tomographic inversion in Seisimager. ( Sorted )Junghee Kim 141
  • Junghee Kim 142
  • Junghee Kim 143
  • Junghee Kim 144